Not OP, and not a very experienced with NixOS (I just use Nix for building containers), but roughly speaking:

* With NixOS, you define the configuration for the entire system in one or a couple .nix files that import each other.

* You can very easily put these .nix files under version control and follow a convention of never leaving the system in a state where you have uncommitted changes.

* See the NixOS/infra repo for an example of managing multiple machines' configurations in a single repo: https://github.com/NixOS/infra/blob/6fecd0f4442ca78ac2e4102c...