Nixos is an os that’s defined by its config stored in .nix files. Everything is defined here all the software and configurations. Two people with the same script will have the exact same os.
Any changes you make that aren’t in the scripts won’t be present when you reboot.
You could maintain a very custom linux distribution (kinda) by just maintaining these config scripts.
So a user wouldn’t need to install all required software and dependencies. They could get a nixos and the self-host config and adjust some settings and have a working system straight after install.