What is the Nix approach to config files?

I understand it allows for stable binaries, without dependency hell, easily installable, possibly multiple versions of the same package; and for easy rollback of the installations. Because code modules are versioned by hashes of their full dependency tree and build options.

But configuration files?

I once installed the Nix package manager on my Ubuntu, and installed some apps with it. Nothing worked. Because some files I have in /etc/ were not compatible with Nix versions of the applications. Those configs were about some deep mechanics of X-related libraries. I never edited them and I don't understand their function.

So, it was impossible to use Nix packages side by side with my Ubuntu. And I suspect the same can happen on plain Nix OS with multiple versions of one package.

Probably I am missing something, but that does not look perfect.

1. For per-user configuration there is home-manager which symlinks dotfiles to Nix store based on a single "home.nix" derivation (state). Home-manager is usable outside of NixOS and, in my opinion, it is better than most dotfiles managers. https://github.com/nix-community/home-manager

2. In NixOS /etc is composed in the same way from the system derivation.

3. Outside of NixOS, I think, one can build systemd units which refer directly to configuration files in Nix store. Probably one can add config paths into wrappers, but it will be very fragile and won't scale.