A breakdown of Scott's moves and which videos they reference (spoiler warning for the lore videos)
Jab - ???
Dash Attack - ???
Forward Tilt - A slash with the Pepto-Bismol sword from Borderline Forever
Up Tilt - Taking his phone off the hook and putting it back (minor nitpick: whenever someone calls him in his videos, he picks up the entire phone, hook and all)
Down Tilt - An enhanced version of Pac-Man's down tilt from SSB4; Scott grew up with Ms. Pac-Man on the Genesis
Forward Smash - A pun on shovelware, which he has done 3 videos on; however, I don't believe he ever actually uses a shovel like this
Up Smash - Tossing the alligator pool toy from Nintendo Before Video Games up into the air
Down Smash - Smashing Lincoln from Chia Pet
Neutral Air - Swinging an N64 controller around
Forward Air - A solid hit with a combination of multiple sports peripherals from Wii Accessories
Back Air - Smashing the New 2DS XL like in Mario Kart 7 | Burning Rubber and Retinas
Up Air - Using a tambourine from the Bourine Bazaar, his tambourine business from Wii Music | Ten Years of Terror
Down Air - Using a basketball as a reference to NBA Baller Beats; note that in the video where he reviews that game (Fitness Games) he uses a watermelon instead. However, this could also be a reference to From Developer to Defunct.
Neutral Special - Tossing a game over his shoulder in disinterest while saying a line from one of his videos. The closest he ever gets to this in his videos is setting multiple RPGs aside at the end of You're Not an RPG Guy: A Scott the Woz Christmas.
Side Special - The knee slap rating scale debuting in The First E3 (1995)
Down Special - The famous "This game blows!" from A Look Back at the Nintendo 3DS
Double Jump: Has him briefly morph into Zip from Fling-Smash (a game he canonically owns at least 25 mysteriously wet copies of)
Taunt - "Now that's what I call a baby!" from Super Mario 3D World | A Critical Second Look
Feel free to correct any mistakes I made or comment the origins of jab and dash attack!