If you watch the videos of The Xerox PARC GUI you quickly see that there was a good nugget of an idea there. But what Apple, Jobs and the crew did with that nugget was an epoch leap of differntiation. From three mouse buttons and a keyboard functioning together to issue commands, down to one mouse button with gestures (click, double-click, click-drag, key-click, etc). The PARC system was a noun-verb input much like the Apollo flight systems. The Mac was much less structured and allowed for far more creativity and approachability to the common user.There was a story on folklore.org about that. Apparently Xerox owned the insurer that Apple used for corporate liability. Xerox didn't sue Apple for infringement because they'd essentially be suing themselves. Go figure, right? And in any case the stuff Hertzfeld actually wrote was orders of magnitude better than anything Xerox had...or anyone else for that matter. And Apple did get some kind of license eventually.
But Apple didn't get any code from Xerox, as far as anyone knows.
You could show a kid a Mac and they could use it, the metaphore of the mouse pointer being your finger was intuative. The PARC system required thought and effort to use.