Minecraft wiki entry for Creating_resource_pack_add-ons, associated text: To rename something, go into the Text file, and find en_us (for English), it is separated into stuff like items and tiles to achievements or warnings
Some mob textures are TGAs, You can get a converter such as Pixillion.
Some other textures (Such as the Sea Lantern,) are animated and are separated into 4 frames vertically.
If some textures like zombified piglin are complicated, try imagining it as a papercraft, or a net, but some parts are in the wrong place (e.g. Top, bottom next to each other)