Comment by kkfx

It's very simple, a basic NixOS config like https://paste2.org/E5IUKfs9 built with

    nix-build '<nixpkgs/nixos>' -A config.system.build.isoImage \
                                -I nixos-config=iso.nix

you can add anything else, for instance X config for interactive usage, perhaps with the same (nearly) EXWM used normally etc. Nix language is obscene but making iso is awesome easy, just a matter of download and build time.