The class hierarchy comes from the overlays which are described in the project file for a project:
The way in which overlays describe their classes can differ from overlay to overlay - it isn’t a trivial mechanism unfortunately. The
Engine-Generated overlay in particular is in one big generated archive file that needs to be parsed to determine the classes. The idea is always to make using Sk easy, though the implementation of the Skookumscript compiler is quite complex.
You can see more details in the the online docs at Overlay files. (You might want to refresh that page to ensure you have the most up-to-date version of the page - I just updated it.)