That's largely correct! To use more precise language, let's discuss two distinct phases: building and activation. Nix will always "build" x, y, and z. But x an

That's largely correct! To use more precise language, let's discuss two distinct phases: building and activation. Nix will always "build" x, y, and z. But x and y might turn into a no-op by being cached in the nix store. System activation is when all those things are symlinked into place. If x, y, and z are, say, systemd services, then there might be some logic that checks if x and y have changed and if the services are already running and decide what to do -- this is probably the most similar to how Ansible works. But this is also a small part of the big picture. Activation is pretty fast even starting from nothing.