Basically you describe the entire system’s configuration in /etc/nixos/configuration.nix, including network, users, packages, dotfiles, etc. Then you run nixos-rebuild switch and boom, your computer is in that state.

i cant say as i havent used it, but i think its supposed to be completely reproducable. i guess sort of like how the aur is basically the universal package repo for arch? the entire system can be configured in text files, so it would be great for installing the same os many times.