One thing I'll give Apple credit for is in the early days of OS X, they had (and perhaps still have, if it's the same thing) a document called Human Interface Guidelines. They actually did real experiments with real people and came up with a set of UI patterns that were proven by science to make software features and UI discoverable, usable, and clear, with the least amount of cognitive load.

Things like, the buttons on a dialog should should be a verb indicating the _action_ the user wants to take. Like "Run This" and "Go Back" instead of "Yes" and "No". (Or worse, the old Windows "OK" and "Cancel", which is rife with ambiguity in so many cases.)

And the tone of the document was that it was intended to be useful to _all_ user interface designers of all software and on all platforms, not just OS X. I just skimmed over the current edition and as far as I can tell, these days it's basically just about how to stay "on brand" with the Apple experience when writing your own UI.