For Day 1, the route you get on the following day depends on the amount of affinity you get for three paths. Those paths being Arim/Chris, Sensei/Lamp, and Keen/Roobit.

On Day 2, the route you are locked into on the following days depends.
For Arim/Chris, it is purely affinity related, with a best of three decision deciding your route.
For Sensei/Lamp, it has one question just for affinity, and one question that outright locks you into one character's route regardless of your previous choice.

For specifically Keen/Roobit, you're mostly locked into Keen's route, with Roobit being difficult to both get into and stay on if you don't know the right choices on Day 3 and 4.

Day 1 - The choices that favour Arim/Chris are Horatio, Throw Lamp in the trash, Set paper on fire, Tip fedora

Day 2 - The choices that favour Arim are Chiselled chin, Free carrot, Murder Chris
From here you are locked into the Arimnaes route.

Day 3 - Below or the choice that give affinity to Arim
-When choosing to attempt small talk, if you choose none of the options and just wait, Arim will say something about sine integers and you get +1 affinity. picking any of the choices gives you nothing
-Successfully using the karate button on keen (using karate on Mr. Arimnaes is -1 point)
-After the karate scene, choose to appeal to Arim's ego
-When asked who the president was in 1960, choosing JFK gives you 2 points, choosing Mr. Arimnaes gives you 1 point
-Choosing basketball for the second activity gives you 1 point. Choosing synchronized swimming sets your points to 0 regardless of what they were prior.
[Fun fact, after you get home on day 3 if you have more than zero points, inner me gives you 100,000 Arim points. Then she accurately tells you exactly how many Arim points you have before removing 100,000 Arim points. Aside from being a gag, this actually tells you exactly how many points you currently have.]

Day 4
-Ask Mr. Arim for his personal record for most time spent peeing
-Confess to murdering someone
-Choose dare when asked for truth or dare

Day 5
-Buy go premium

Ending 1: Performance MOde
On Day 5, have 6 or more affinity and choose "Yes it is tho".

Ending 2: Energy-Saving
On Day 5, choose "WHat about Xmas". "Yes it is tho" also works if you have less than 6 affinity.

Ending 3: Low Life
This one is a bit more difficult to get. You must get to Day 5 with 0-2 affinity with Mr. Arimnaes to unlock a third dialogue option in a choice at the end.
This is easier if you pick "synchronized swimming" on Day 3 during the sleep over because it resets your affinity with Arimnaes to 0 (possibly permanently for the rest of the run, though I'm unsure).
When you meet the requirements, you will be presented with a new third option "♥♥♥♥ u mr arim".

Day 1 - The choices that favour Arim/Chris are Horatio, Throw Lamp in the trash, Set paper on fire, Tip fedora

Day 2 - The choices that favour Chris are Blue eyes, Poetry slam, Confess
You are locked into the Chris route, with one exception regarding a single specific choice on Day 4.

Ending 1: Well Done
You must defeat Sav in his boss fight. You can buy stuff at the Twist Shop one last time on Day 5. Buy the Cool Guy Dictionary if you can (because that instantly kills Sav). If Lamp tells you that your "cool guy points" or whatever are too low, just buy other items in the shop to increase these points and then get the Cool Guy Dictionary.
Use the Dictionary on Sav.

Ending 2: Medium-Rare
Lose to Sav in his boss fight.

Ending 3: Blue
On day 4, when given the prompt choose "This is the Sensei route".
Continue as normal, and on Day 5 when you get to choose one of Sensei's family members, to talk to Plungersei. (This technically also counts as a Sensei ending since it goes through part of his route, and once you obtain this ending, it colours him on the title screen.)

Day 1 - The choices that favour Sensei/Lamp are Something cool, Accept and Smooch, Creative freeform, Give advice

Day 2 - The choice that gives affinity for Sensei is The library. The choice that locks you into Sensei is Save Sensei
From here you are locked into the Sensei route.

Day 3 - Below are the choices that let you gain affinity to Sensei
-Choosing pineapple slices as the appetiser
-Choosing oatmeal as the main course
-Choosing encouragement on brain's first question
-Choosing green on brain's second question
-Choosing barbara streisan on brain's final question

Day 4
-Choosing to write your name on the test
-Choosing to learn about planets when planning the party

Day 5
-Who you decide to talk to at the party on day 5 matters, and can give you varying points depending on who you talk to and what you say to some of them. Talking to Ricardo however is the best choice as it automatically gives you 2 points

Ending 1: Black Tie
Get 6 or more affinity points with Sensei.

Ending 2: Business Casual
Get below 6 affinity with Sensei.

Ending 3: Streetwear
On Day 5, when given the choice to speak to one of Sensei's family members, choose Cousin Stunkel. Then do not forgive him for his pun.

Worth noting that Chris' Ending 3 also counts towards Sensei as both their portraits are filled in the title screen, refer to the Chris section of the guide.

Day 1 - The choices that favour Sernsei/Lamp are Something cool, Accept and Smooch, Creative freeform, Give advice

Day 2 - The choice that gives affinity for Lamp is Ironic skateboard park. The choice that locks you into Lamp is Listen to Lamp
From here you are locked into Lamp's route.

Ending 1: Slam Dunk
On Day 5, choose "shoot (bball)" in the final round. This choice only shows up when you get all 5 dunks in the basketball QTE, or if you have QTEs disabled.

Ending 2: Basket
On Day 5, choose any option besides "shoot (bball)" in the final round.

Ending 3: Airball
When you get the option to do so, reject her on Day 4 or 5.

Day 1 - The choices that favour Keen/Roobit are Something non-threatening, Politely decline, Give best effort, Give 3 dollars

Day 2 - There is one important instance regarding Sav if you want Keen's bad ending. Refer to Ending 3 for more info on that. Otherwise, below is how to get affinity with Keen.
-Saying you are not busy
-Believing keen is innocent
-Saying hello to Sav
-Choosing the size as the incorrect order detail (choosing quantity gives you minus 1 point here, cheese is neutral)
-Choosing minestrone as the incorrect order detail about the salad (choosing "forgot to apologize" gives you minus 1 point here, dressing is neutral)
-Choosing to ring up Sav's total at the end of the his interaction gives you 1 point BUT choosing to smile gives you 2 points (alternatively if you give Sav a 5 star review for his pizza you gain 1 point, but you miss out on the other gains)

Day 3 - Avoid choices that focus on the murder mystery/Roobit as you want to get Keen's endings, and pick choices that raise or lower Keen's affinity depending on which ending you're going for.
From here you are locked into Keenarnor's route.
-Choosing to tap dance to brain
-Choosing to ask keen during roobit's investigation

Ending 1: Classic
Gain 5 or more affinity with Keenarnor by Day 5.

Ending 2: Diet
Gain below 5 affinity with Keenarnor by Day 5.

Ending 3: Zero
On Day 2, select Shooting Sav. Once Sav is dead, but before you Game Over, rewind to "undo" what just happened, select a different choice and continue as normal. On Day 5, you can confess that you were Sav all along. I am not sure if this is exclusive to the high affinity version of this route or not, but I believe it is.

Heads up, Roobit's route is the hardest to both get in and stay in as she requires picking the right choices on both Day 3 and 4, otherwise you will default back to the Keenarnor route.

Day 1 - The choices that favour Keen/Roobit are Something non-threatening, Politely decline, Give best effort, Give 3 dollars

Day 2 - It's exclusively choices that give affinity to Keen, so pick anything you want as it does not affect Roobit.

Day 3 - At the start of keen's day 3, you have 0 Roobit points and 0 mystery points. You need at least 2 Roobit points AND 2 mystery points at the end of day 3 to get onto the Roobit route.
-Choose business card as the clue on for 1 Roob point AND 1 mystery point. Then say the gumball machine for another 1 Roob point. OR if you choose roobit is a clue 3 times, you get 2 Roob points, but miss out on the mystery point.
-Choose "it wont work" for 2 mystery points, or "look for rubbing alcohol" gives 1 mystery point.
-Choose "waking up neighbor with shoe" as the origin of the phrase for 1 Roob point.

Day 4 -
-Giving Roobit a gumball
-Ask Keen about the victim for 2 mystery points, or his alibi for 1 point
-Ask Boo about having to pee for 2 points, or eating from the garbage for 1 point
-Ask Arim about his alibi for 2 points, or about his order for 1 point
[At this point you need 6 mystery points or you will drop back to the Keen route]
-Pick that Sensei was killed outside dumpster on day 3
-Identify Arimnaes as the murderer
-If you have 4 or more points you will get a choice on day 4 to say that you are besties, this gives you +1 point

Ending 1: Express
On Day 5, if you have 5 or more affinity, you get the option to choose "I LoVE YOU !"

Ending 2: Standard
On Day 5, choose "I Miss You"

Ending 3: Ground
On Day 4, choose to get Roobit a gumball. She will give you a quarter.
On Day 5, choose "I never paid you back!"

The True route is attainable after you get at least one ending for each love interest. From here, at the beginning of every new playthrough a new choice will show up at a certain point. You can choose to continue to the normal routes, or play the True route. I have spoilered everything below so as to not accidentally show spoilers for anyone who has not gotten to do a blind run of this route. Trust me, it's genuinely amazing.

How to save (or not save) all the love interests before Day 5

Day 1
-Take a picture of the severed head on the printer - This keeps Roobit safe as you'll have a piece of evidence to hand in to the police. If you don't do this, then on Day 2, Roobit will go to Mr. Arimnaes' office herself and will either die if you don't go there, or result in a choice between sacrificing her or sacrificing yourself (which leads to a game over sequence that is uniquely emotional compared to the other ones).

Day 2
-When talking about Mr. Arimnaes with Keenarnor, tell him "Run away" - If you choose the other option, Keen will die off screen.

Day 3
-Save Chris's life - This one is pretty self explanatory.

Day 4
-Poem guy - Choosing the other option will lead to Lamp's death.
-Save Sensei - Also pretty self explanatory.

Keeping everyone alive against MOTHER

Keenarnor - Choose Not on the menu, Didn't ask for bottle or draft, She doesn't work there
Roobit - First choice look at the scrapbook and click on MOTHER's profile. Then the option Tortilla should appear. Second, when the screen turns black, take a photo. Finally type "no".
Chris - (I don't know if this selection of answers is perfect, but it does keep Chris alive just fine) Fields of gran in the distnace
Let’s do a dance together.
Only together are we strongst
Real life is beter then dreams.
Do you remember our first kiss?
Also somthing about vampires?
Do NOT disturb the fish.
Daisies. Daffodils. Delaware.
Little acievemnts every day.
End on a high note!
Warped Lamp - Choose The coolest spots to hang?, Rate Flordaddles coolness, Its ironic
Sensei - In the trash he goes!
Mr. Arimnaes - Select all the answers until none are left.
Kiane - Any answer is fine.

The Bad ending

WARNING: Permanent Game Over if you try this ending. Once done, you will be permanently unable to play the game unless you wipe all of your save data. So be warned, after finishing this there's no going back. it also isn't necessary for 100% achievements, so there's no need to bother with this ending.

After you have played the True Route once, near the start you get an option to say if you love or hate pizza game. Say that you want Pizza Game to end. From here, you'll want to kill everyone.
-Follow what I mentioned in the pre-Day 5 section and do the opposite.
-To kill Arimnaes, when given the prompt to "Stop Her", simply wait for a few seconds instead and MOTHER will kill him. I believe this prompt only appears when the other love interests are dead.

After everyone is dead, eventually Mother will point out that you don't care for anyone and asks if you want Pizza Game to end. You are given several chances to not give in here and avoid completing the bad ending in case you don't want to permanently ruin the game. The point of no return is when Stabby says the line "I'm going to make the world a better place, just for you". After this line of dialogue, you will be locked out of playing game forever unless you reset your save data.

At the end of the game, you get a scene with the character you have the highest affinity with. (Dead characters don't count, obviously). If two or more are tied, you instead get the Stabby scene. If your highest is Sensei, you get a brief scene with Sensei, and then you get the Stabby scene instead.

All true route decisions in order:

Aren't you sick of all this?
"Yes, I am stick of it": Chris, lamp, Roobit
"No, I am sick of it": Arim, Sensei, Keen

Oh? What did you witness?
"A severed head": Roobit, Keen
"His mean words": Sensei, Lamp
"Suspicious artwork": Arim, Chris

I' dlike to ask you to cmoe down to our headquarters to talk more about this and give a statement, if that okay?
"Yes": Chris, Sensei, Roobit
"No": Arim, Lamp, Keen

Kiane, can you tell me what the day after tomorrow is?
Well... that would be...
"Your birthdays?": Keen, Sensei
"A jewish holiday?": Chris, Lamp
"October 32nd?": Arim, Roobit

Question 1: Are you wilfully self-destrictive?
"Yes": Arim, Keen, Lamp
"No": Roobit, Sensei, Chris

Question 2: Are iyou still under the DELUSION that there is going to be some sort of relationship between us?
"Yes": Sensei, Chris, Keen
"No": Roobit, Arim, Lamp

Are you ready to DIE for your GODEMPEROR?
"Yes" 1: Arim, Sensei
"Yes" 2: Chris, Keen
"Yes" 3: Roobit, Lamp

Its worse.
"Sounds cool!": Sensei, Keen
"wow thats dumb": Lamp, Arim
"blinks silently": Roobit, Chris

(Cool, What should I do to convince Lamp to loawyer for me?_)
"Kiss": Lamp, Sensei
"Give a speech": Chris, Keen
"Threaten": Arim, Roobit

You should call me something else too, so Mr. ARim doesn't know I am police man instead of regular!
"Call him Sweetie": Chris, Lamp
"Call him Jay": Roobit, Keen
"Call him late 4 dinner": Arim, Sensei

Final dating round! Don't worry, it's easy!
"Save Chris's life": Chris, Keen, Sensei
"Arrest Arim": Arim, Roobit, Lamp

[At the start of day 4, which dream you get depends on who has the highest points. if it's Arim, Lamp or Roobit, you get the Arim dream. If it's Keen, Sensei or Chris, you get the dream about the call with your mom]

What should I ask Chris about?
"Mr. Arimnaes": Arim, Roobit
"The poem guy": Chris, Lamp
"Sensei": Sensei, Keen

maybe he had a problemw with...
"Something you done?": Roobit, Keen
"Who you are?": Arim, Chris
"The poetry? slam?": Lamp, Sensei

Can you oforigvie me Kiane?
"Yes": Sensei, Chris, Roobit
"no!! grrr": Arim, Lamp, Keen

Maybe... Maybe he just likes murder?
"Your probably right": Lamp, Keen, Sensei
"No, thats wrong": Arim, Roobit, Chris

The only person who would do something THIS radical is...
"Roobit": Chris, Roobit, Keen
"Lamp": Lamp, Arim, Sensei

Your right, we havent done or said anything that might get him inadvertendtly killed yet!
"Obliterate Sensei": Arim, Lamp, Chris
"Save Sensei": Sensei, Roobit, Keen

Hmmmm. if i Had to guess when Mr. Arim was a baby,I'd say...
"1977-1981": Arim, Chris
"Lincoln Presidency": Sensei, Keen
"Y2K": Roobit, Lamp

Which choice you beat Mr. Arim with also gives you points. It only counts if you win the debate with that choice, so you can't get more than one of these by deliberately failing some first.
"I sabotaged it": Arim, Keen
"You're powerless": Chris, Sensei
"Women are unkillable": Lamp, Roobit

If Willibean his beacons out of people he knew but was not close to, then it would make sense that the last beacon is...
"Brain": Lamp, Roobit
"Me again!": Sensei, Keen
"Mr. Arimnaes": Arim, Chris

lets see.. The most important thing we experiensed here in Flordaddle... that would have to be...
"Friendship": Chris, Lamp
"Elevaters": Arim, Lamp
"Keenarnor" (only if alive): Keen, Roobit
"Mr. Arimnaes" (if Keenarnor died): Arim
"Pizza": Roobit

https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=1867804005 https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=1882610268 https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=1948996284
Each of these guides helped me with figuring out the paths for each ending and how they worked, and I am thankful for them. I would also like to thank the people in the Pizza Game discord server who assisted me when I asked questions about this game's routes. Y'all were amazing. There was also the wiki, however that thing is horrifically unfinished for some of its character pages. Still, it did help in piecing together how most of these paths worked.
[Update: A member from the Pizza Game discord named Atrum also contacted me a few months after posting this guide, and helped me add in all the extra knowledge of affinity that they could find in the game's code. Special thanks to them.]

Honestly, making this guide was a lot of fun. I hope it helps you out if you're looking for a particular ending. I don't know if I'll ever touch this game again but I will say my time with it was nice, even if I now need to repair my vocabulary and the way my internal monologue speaks.