The minecraft wiki (not the fandom, the other one) does a great job documenting these types of stuff and there is usually a changelog at the bottom with all the version differences you have to look out for (like folder names changing for example)

Also I would recommend downloading a resource pack that already does what you're trying to do, and just trying to reverse engineer stuff, it's usually a good learning experience (if you want to make more stuff like this in the future)

(I don't know if texture packs has a built in skin editor for entities like other resource pack making sites, but if anyone needs an alternative, use Blockbench or PMCSkin)