Those are handled by NixOS - that's the purpose of NixOS, it was built on top of Nix explicitly to manage such things. You can read some about NixOS at https://nixos.org/nixos/about.html
And in particular, NixOS is a full OS, like Arch/Ubuntu/etc. You can't install NixOS side-by-side with Arch any more than you can install Arch side-by-side with Ubuntu. Nix-sans-NixOS is only the stuff that can be installed side-by-side with an existing OS - no service management, no kernels, etc.