I don't intend to maintain an up-to-date source of data, this guide is only about how healing works which is unlikely to change. Consider this a PSA on how players should interact with heals

I hand-tested all of it

Edit 9/2024: rephrased a bit, does not necessarily reflect the current game (e.g. I heard Melfi Heart normal REPAIR ARROW is revamped so no more of those shenanigans)

AFAIK there are 3 categories of heals

• "Active" heals -
  These are the most common heals that pop up an icon on the recipient
  A caveat that is never explained is there's a roughly 0.5s CD on the receiving end for these heals, other active heals during CD are ignored/nullified
  e.g. Medicball "REPAIR GUN", WB "Repair Unit", Priestol Q "REPAIR FIELD"
  
  
• "Passive" Auto Repair heals -
  These are hp regeneration effects applied like a buff, there can only be 1 active Auto Repair effect it requires the previous one to end for the next one to apply and they cannot be extended. There are exceptions though like Medicball "PICK UP" which overrides existing Auto Repair
  While most of them seem to tick once per sec, some seem to have slightly varied tick rate, and at least one ticks exactly every 0.5s
  e.g. Regina Winberrl "REPAIR FIELD", Medicball "PICK UP", Medi Aide "Control Repair"
  
  
• Others-
  These are the rarer ones that don't belong to the 2 main groups above. At least on the healing portion they don't seem to have any restrictions so they can be applied freely
  Buff granting healing bubbles that have no pop-up icon apparently do belong here and do not conflict with Repair Unit etc.
  e.g. Amy "REPAIR MEDICINE", Hilti "REPAIR RESTORE", Mediger Sign "MAGIC BOOK"

As long as you play by the category rules, multiple heals can be applied to the same player at the same time

On the contrary, this is why Repair Unit is generally despised, it heals slowly yet disrupts other heals by occupying the 0.5s CD

There are a few ways that a heal applies to the player. These are just how I named them

Just a brief reference table, more useful to those seeking healing

Be reminded however that most types here are sensitive to hit registration, so Projectiles can be eaten by the terrain and Fields may not heal if the elevation doesn't match
Below are two bots to be used with WB "Repair Bit". The left will miss a leveled Zero Saber while the right won't. The difference is the left one has a taller head hence Repair Bit placed higher

Sorry I'm just here to talk about the mechanics so no build suggestion or healing data from me
Hope you can figure out why not to stack Repair Units by now

Here's a video for some of the interactions mentioned

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There are however some notable shenanigans regarding healing that may or may not be intended so they go into this section

• Repair Bit/Unit X Hypershot -
  For some reason, applying Hypershot on Repair Bit/Unit alters their tick rate
  Repair Bit drops from 18 ticks/10s to 15 ticks/10s, losing ~16.5% healing rate/total
  Repair Unit raises from 54 ticks/43s to 75 ticks/43s (8 str/tec), gaining ~39% healing rate/total
  This however does not make Repair Unit any better than other healing options, only even with the weaker ones
  
  
• Medicball X Repair Bit/Unit
  Repair Bit/Unit doesn't heal the rider on Medicball. This only affects the ridden medicball
  
  
• Timer Repair X Auto Repair buff -
  When a player has Auto Repair buff active, Timer Repair "buff" from Amy/Medicarest does not apply (heal as normal)
  This is technically not about healing, but this is quite significant so it's worth mentioning
  
  
• Auto Repair overriding -
  When a player has Auto Repair buff active, using Ride type healing overrides the Auto Repair tick to that of the Ride type, persists until the Auto Repair wears off
  e.g. Take Regina Winberrl's 15s Auto Repair, gets on Amy's 30hp/tick Healing Bed, gets off, enjoy ~12s of 30hp/tick Auto Repair
  
  Ultimately doesn't sound very practical. Though it indicates that Ride type healing is categorized as (passive) Auto Repair heals despite not exactly the same
  
  
• Auto Repair priority -
  While Melfi Heart's charged REPAIR ARROW behaves like normal Auto Repair DriveThrough bubble, her non-charged REPAIR ARROW has a property that essentially gives it the highest priority in Auto Repair heals
  It
  1. overwrites existing Auto Repair heals
     
  2. prevents other Auto Repair heals from applying (which is normal)
     
  3. can overwrite itself which means it can extend its effect indefinitely
     
  4. overrides Ride type heals to its 7hp/tick, persists until the riding is cancelled
  
  This can be very disruptive and I hope no one is trolling with this info :L
  
  Also, charged REPAIR ARROW's projectile actually acts like non-charged with very short range, careful with how you apply it