it's not on top of Nix itself but (about to be) a part of it. according to the NixOS Wiki [1] Flakes is an upcoming feature of the base package manager. it's not part of the stable channel yet, and "proper" documentation only currently exists in the unstable manual [2].
[1] https://nixos.wiki/wiki/Flakes [2] https://nixos.org/manual/nix/unstable/command-ref/new-cli/ni...