Impermanence in NixOS allows for the wiping of the root directory every reboot by mounting a tmpfs to it, with only /boot and /nix needed for booting. The rest of the system files are just links to files in /nix. Persisting certain files and folders between reboots can be achieved through bind mounts or the NixOS Impermanence module. Home partitions can also be made and mounted as normal for persistence. The Home Manager Impermanence module allows for the persistence of files in the home directory while still being writable. A troubleshooting issue may occur if NixOS and Home-Manager versions are mismatched. Surprising content includes the use of ZFS snapshots instead of tmpfs for this purpose.
https://nixos.wiki/wiki/Impermanence