That's auto pairing of new devices, using the Android Auto app to send regular bluetooth audio to the car. Android Auto when plugged in disables the phone's screen and all functions now operate on the car's infotainment screen. Bluetooth is used for two way communication but USB is used for charging and the video signal. This is not like CarPlay which does have wireless capabilities in some vehicles because the CarPlay app runs primarily on the vehicle's hardware, not the phone's.