If you want to contribute to nix the package manager (which I'm really not sure you want to if you only understand the basics), you can look at the contributing guide here:
https://github.com/NixOS/nix/blob/master/CONTRIBUTING.md

If you don't want to work with the (not so great) C++ code of nix the package manager you can go and contribute to nixpkgs:
https://github.com/NixOS/nixpkgs/blob/master/CONTRIBUTING.md
You can help with fixing reproducibility of a package, add a new one, write a nixos module for a missing program/service or work on more fundamental stuff like the stdenv or any of the language ecosystems. Personally, these two are interesting for me, but you might wanna search yourself:
https://github.com/NixOS/nixpkgs/issues/334195
https://github.com/NixOS/nixpkgs/issues/333702

If you don't want to write nix either, there are outside issues like
https://github.com/NixOS/nixpkgs/issues/424345
decentralized binary caching, etc

You can also help around on discourse or in #users:nixos.org