It can cache at the level of any set of files, technically speaking (a single file, directory of files, and so on); so you could even use it as a Makefile repla

It can cache at the level of any set of files, technically speaking (a single file, directory of files, and so on); so you could even use it as a Makefile replacement or something for example. But most people don't do that; the ecosystem is broadly much more coarse-grained and designed around "packages", yes. I wasn't really referring to the coarseness though; just that a lot of Nix projects provide build caches to speed things up for contributors. It's not just something for internal teams. And it really does help.