>>381000031
Udon is technically not C#, but there is UdonSharp which compiles C# to raw Udon assembly. It's very much like neutered C# (absolutely no lists, dictionaries or other data structures, arrays ONLY! Also very few custom classes are possible so leave that OOP stuff at home for now).
But that's just for making worlds.
As for actual modding, you can do it entirely in C# but it's hard to mod the game as everything is obfuscated.
Avatar development just uses plain Unity, very little scripting is involved. In any case, having C# and Unity knowledge will give you a big headstart on just about everything.