Yes, in Nix code it is possible to refer to arbitrary Nix codebases from Nix code.
I'd point to the home-manager installation instructions for an example of Nix flakes having a nicer UX. Running flake-enabled codebases is just much nicer than non-flake codebases.
The former involves running `nix-channel --add <...> home-manager && nix-channel --update` in order for `<home-manager>` to mean anything. With the latter, it's simply `nix run home-manager/release-23.05 -- init --switch`. (Albeit, `home-manager` is already registered as an alias for `github:nix-community/home-manager`).
One key detail is that `nix-channel --add` modifies the 'system state', whereas running flakes essentially does not.
-- Regarding "you can pull in arbitrary Nix code by reference". With flakes, you have a standard entry point where you know the interface it has. Whereas, having a 'default.nix' is a convention, and there's no way to know how a `default.nix` is to be used without reading its source.
