Conceptually, Nix is just the next evolution of tools like Ansible, and tangentially related to projects like Silverblue, but in practice, it's only used by enthusiasts. And, of course, you can use Nix outside of NixOS.
Unless there's a tool I don't know about, there's no equivalent for Discover or Gnome Software for NixOS. Because that's the class of boring people that make up the silent majority: the people who don't know how to, or don't want to edit configuration files. This was how Valve made Linux on a console a success, and it's why Ubuntu is still popular despite their experiments causing them to be decried by the community over and over again.