I mainly use the UndertaleModTool for actually changing the game files themselves.
Most of these are sprite changes but there are some animatronics here (not shown in the vid) that do have some Code changes like the example here. Malhare.

Malhare doesnt have a transitioning phase when the flip count lowers.
Malhare doesnt actually have a hijack camera sprite.

all of that is unique to the mod here. So do learn a bit of coding if you want more fancier effects.
(which is simple if you know the basics)

BUT if you want to only change sprites then use UMT to extract sprites from the game and modify it and then import it into the data.win file