For managing your configuration.nix file itself you can just use whichever VCS you want, it's a text file that describes one system configuration and managing multiple versions and snapshots within that configuration file is out of scope.

For the system itself, each time you run "nixos-rebuild switch" it builds a system out of your configuration.nix, including an activation script which sets environment variables and symlinks and stops and starts services and so on, adds this new system to the grub menu, and runs the activation script. It specifically doesn't delete any of your old stuff from the nix store or grub menu, including all your older versions of packages, and your old activation scripts. So if your new system is borked you can just boot into a previous one.