The user side is configured in /home/<user>.

To set up a user configuration it must be declared in /modules/base/users/options.nix and /modules/base/users/<user>.nix can be used for anything that is not preconfigured by /modules/base/users/mkusers.nix. You will also want to add a hashed password generated from mkpasswd to /modules/nixos/users/<user>.nix.

You should also add your user into the users.nix file for your system. Whilst following the template:

{
  garden.system = {
    mainUser = "kitty";
    users = [ "kitty" ];
  };

  # you can add a home-manager configuration here for the user if it needs
  # anything special
  home-manager.users.kitty.garden = {};
}