When it comes to building derivations, yes. Which describes almost everything you can do. For live system rebuilds in NixOS, the final step involves examining

When it comes to building derivations, yes. Which describes almost everything you can do. For live system rebuilds in NixOS, the final step involves examining the system to decide which systemd services need restarting and restarting only those; that’s part of the process called activation. But that’s the only exception, and doesn’t happen if you reboot instead.