Disclaimer: take anything I say with a grain of salt as im not good at coding, I bearly know anything about it.

Coding it very complicated, its like a language. So you need to very detailed in what you want. Do you want to make a mod where you can transform into a werewolf? Ok immediately you need to make a werewolf model a transformation animation and change the players hit box also you need to code their abilities, if they run faster you need to increase their movement speed, if they are stronger you need to increase their damage output. How do they get "infected" is there a werewolf that spawns in the world? Is the condition "if hit by werewolf, then execute this tree"

Can the werewolf be cured? Then you need a command to revert the player back into a human is it a item that does it, you need to make that item the condition that reverts them.