NixOS is really cool! I've actually tried it out a couple of times, and I find the premise of a completely reproducible system to be incredibly intriguing. In the time that I've tried it, I've found that it strays a bit too close to the "The way this system is designed is getting in the way of the things I want or need to do with my PC" - but it will up there on the distros to circle back to every now and then to see if things change.

You're correct that Nix is a few different things, it's a language (the Nix language), a package manager (Nix as a package manager), and then finally a distro as well (NixOS) - when running NixOS you get the benefits of all of them wrapped up in a convenient package so that your whole system can be managed in a declarative way (even down to your home folder / "dot files" if you use Home Manager - which technically also works outside of NixOS too).

Your knowledge of the FOSS world is stellar, even if you're not in the tech field - keep up the great work because I can tell you've got some wonderful adventures ahead of you!