Obviuosly (or not?), this is something completely unrelated to operating system. It seems Mac Mini's hardware includes keyboard and mouse already plugged as HID devices onto its internal USB hub. Only that they aren't plugged, and come over the air instead.

It's the Bluettoth radio that's plugged in. Clever. But actually, it's something Apple is forced to do, if they want to sell a computer with Bluettoth input devices, that have to work BEFORE the operating system has started, it the firmeare.