I'd say NixOS's declarative config more/less covers the config files, where on non-NixOS you'd edit the files under /etc (and hope you didn't break anything).

Whereas, Home Manager's declarative config is more/less the stuff underneath $HOME or the XDG config dir.

Turns out that a few things with fish shell don't work with a purely declarative config. (e.g. the universal environment variables are stateful).

For myself, I don't rely on writing all my configuration in Nix files. I configure fish shell using config.fish. (Though I do use HM to symlink the config file).

Ie with Home Manager, i just have Nix copy (link) my config files to the place apps look for them. I only would even consider recreating them in Nix if i needed programmatic construction of them or if i was trying to distribute them as a Nix package or something.

But for just me, and my own use - i see zero reason to recreate config files which are already managed by Nix. It's just extra work with no benefit as far as i can tell. Especially because i detest the Nix language. It also makes it very difficult to search for, because if you want to figure out how to configure X feature of Y app, you need to search for that and then mentally map it to Z Nix configuration which may or may not support it. It's just layers of needless abstraction for a single-use configuration.