Ah, my apologies, I think I've misunderstood you. Yes, absolutely, I agree - reading definitions of those modules is how one learns Nix. Documentation is okay, but it doesn't cover neither the essential theory like patterns, practices and idiomatic approaches, neither implementation specifics (how some particular config file or startup helper script is generated).
Although, to be fair - reading source code of how things work is not really specific to Nix. It's just that Nix doesn't stop at packaging and also provides declarative configuration, so there's much more to read in nixpkgs than inside some .deb source (which typically stops at providing a generic systemd unit or rc script). But if someone would be using some other declarative configuration tooling - I'm sure they'll also do a lot of deep diving into the definitions, reading the source code. It's just that Nix is outstanding in this regard - I suppose I can, but I surely wouldn't want to use Terraform, Chef or Ansible for what it does.
Although, to be fair - reading source code of how things work is not really specific to Nix. It's just that Nix doesn't stop at packaging and also provides declarative configuration, so there's much more to read in nixpkgs than inside some .deb source (which typically stops at providing a generic systemd unit or rc script). But if someone would be using some other declarative configuration tooling - I'm sure they'll also do a lot of deep diving into the definitions, reading the source code. It's just that Nix is outstanding in this regard - I suppose I can, but I surely wouldn't want to use Terraform, Chef or Ansible for what it does.