If you want to build custom images just use https://github.com/nix-community/nixos-generators
It's documentation is on point.
Would it be possible to add a minimalistic image that contains app, kernel and essential packages? Like what you would get when building a docker container or using the not-os repo that I mentioned earlier.
I think having mini images would be awesome.
You might like: https://nixery.dev/
e.g. if you want a Nix container with a shell environment, git, and htop:
docker run -ti nixery.dev/shell/git/htop bash
There are efforts to improve documentation, but it still is lacking (I think the biggest problem is that Nix is so big, not just the OS but it can be utilized as a build system).
Just with NixOS is not exactly clear how can you for example build your custom image.
I think https://nix.dev/ is approaching the documentation from the right direction.
There are also many pieces that people built that you need to find.
For example some things that I found accidentally:
* https://github.com/nix-community/poetry2nix
* https://github.com/matthewbauer/nix-bundle
* https://github.com/cleverca22/not-os
Unfortunately those side projects often have even worse documentation.