It is best practice to assume the host configurations make use of the link:../modules/nixos[custom NixOS modules], link:../pkgs[custom packages], and the flake inputs.
In other words, always pay attention to link:../flake.nix[`../flake.nix`].
* You could also easily map link:../users/home-manager[one of my home-manager configurations] into one of the users for a NixOS system with `lib.mapHomeManagerUser` which accepts two arguments: a name from of the home-manager user folder and the user config as if configuration with `users.users.<name>`.
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Here's an example to easily get my main home-manager config to be one of the users of the system.