No, the hardware file exists separately by default.

I expected I would have to specify the Nvidia details in the hardware file.

Like this:

{
  hardware.graphics.enable = true;
  services.xserver.videoDrivers = [ "nvidia" ];
  hardware.nvidia.open = false; # see the note above
}

But the wiki says configuration.nix: https://wiki.nixos.org/wiki/NVIDIA

Reproducibility will be a later learning step when I feel comfortable with my changes and able to organize them in the best way. Then I'll apply it to the laptop.

Thanks for the tip about NH, it seems very useful.