As soon as you write an USB controller driver, you'll stumble over that quickly. The EHCI and XHCI specs are freely available and describe this in detail. It's just that only few people need to write such drivers. :-)

As for it working so well, that's mostly on the EE side: they were _very_ careful to provide this hardware based routing with defaults that make sure that old OSes continue to work while new OSes (or new drivers for old OSes) can make use of the new controllers.