Nix itself is in first approximation "just a package manager"

I'd go even further: Nix is just a build tool, like Make.

In fact, it assumes far less about the system than Make; e.g. Make does all sorts of shell-related things, whilst Nix simply executes a binary (with a list of arguments and a set of env vars).

(Nixpkgs definitions almost always use a bash binary as their executable, so it feels more like Make; but Nix doesn't care)