Advanced modding need help

I'm new to modding Ive learned how to make maps and change textures and sprites but I want to learn how to do more advanced stuff like add rpg elements like an inventory etc

You'll have better luck on the zdoom forums.  For custom inventory items you need to use decorate.  You can look at any mod that has such code and learn by looking, that's how most people learn I expect.

https://zdoom.org/wiki/Classes:CustomInventory

8 hours ago, BaphomatBob said:

I'm new to modding Ive learned how to make maps and change textures and sprites but I want to learn how to do more advanced stuff like add rpg elements like an inventory etc

In addition to Bryan T's comment, you may want to check out the Doom Editing subforum here. There may be some threads that will be useful to you.