Nix / NixOS

The issue I see with your approach is that you’re looking for a tutorial for something that is basically infinitely flexible, there’s multiple ways to do things, etc.

I don't think this is the right approach either TBH. Even general-purpose programming languages often have "tutorials" to guide you through the basics of writing your first couple of programs, explaining all the steps without showering you with too much details about internals. Think Rust By Example or Real World Haskell. I feel like NixOS urgently needs something like this - a half-dozen (relatively) simple configuration examples, from the shortest one possible that gives you the minimal useable system, all the way to a flake-based setup that configures multiple machines, both headless and GUI-enabled, with appropriate code reuse. Don't get me wrong, those resources are there if you look for them, someone just has to compile them and get them into official docs.