NixOS modules are built around Nix and systemd so theoretically you could write a port for a different GNU/Linux distribution if you have those available. I'm not aware of any though. There is however a variant for Darwin based on launchd: https://github.com/LnL7/nix-darwin

You can find the implementation of the nginx service here: https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/s...