To my knowledge, there are 3 main types of Android car USB audio:
1. Android device as storage device; car audio system as player - DAC - amp - speakers
2. Android device as storage device - player; standard USB DAC; car audio system as amp - speakers
3. Android device as storage device - player; car audio system as DAC - amp - speakers.
1. Android device as storage device; car audio system as player - DAC - amp - speakers:
Android phone as storage device >> regular USB cable >> car audio system: car audio player > car audio DAC > car audio amp > car audio speakers
The car audio system operating in USB Host mode provides power to the Android phone operating in USB Device mode.
Using your Android phone with your car stereo (How To) - CNET
2. Android device as storage device - player; standard USB DAC; car audio system as amp - speakers:
Android phone as storage device - player(s) >> USB OTG cable >> compatible USB DAC >> car audio system: car audio amp > car audio speakers
The Android phone operating in USB Host mode provides power to the standard USB DAC.
As often as possible, a USB OTG/ACA cable should be used so that the USB ACA-featured Android phone operating in USB Host mode is also able to be charged by a power supply at the same time.
Android phones and USB DACs - Page 363
3. Android device as storage device - player; car audio system as DAC - amp - speakers:
Android phone as storage device - player(s) >> regular USB cable >> car audio system: car audio DAC > car audio amp > car audio speakers
The car audio system operating in USB Host mode provides power to the Android phone.
This car audio DAC (i.e. Android-specific USB DAC) is obviously not compatible with PC or Mac. It is compliant with the Android Open Accessory protocol 2.0 (AOA 2.0).
Android phones and USB DACs - Page 155
Android Compatibility | Pioneer Electronics USA
Firmware Updates | JVC
Standard USB audio supported by Android 5.0 Lollipop:
Android phones and USB DACs - Page 377