Ah I see. But what are they a feature of, then? The nix store?
I would consider derivations and builtins to be language features. Where would you expect them to be documented instead?
I think what they’re most a feature of is the nix-daemon which generally facilitates builds and substitutions.
How exactly this works isn’t documented and that’s been pointed out as something you could do for a sponsored project even.
Under “Nix Language & Features”.