I mostly just searched nixos how to package pyrhon/go/rust/ program or nixos how to package sddm theme/gtk/...

The best resource honestly are the randon blogposts since the wiki itself is really bad.

I also recommend the channel vimjoyer.

I also recommend to get into the habit of searching for options on https://search.nixos.org/options and for packages on https://search.nixos.org/packages which are great resources to know what you can set or install and already packages.

You can also check my nixos config on examples for how to package sddm theme and shell scripts.

I also have a couple programs on my selfhosted gitea that use flakes for packaging which you can checkout also.

https://code.cronyakatsuki.xyz/crony/nixos