These are the ones that are auto-installed without tapping and it makes
sense to make them more prominent in our documentation as we've
offloaded various functionality over the years to each of them.
This is the (partially) documented public API for Homebrew.
The main class you should look at is the {Formula} class (and classes linked from there). That's the class that's used to create Homebrew formulae (i.e. package descriptions). Assume anything else you stumble upon is private.