I think you’re over thinking it. The simple description of how this likely works is: - the tracks are encoded with Dolby atmos that allows encoding positional

I think you’re over thinking it. The simple description of how this likely works is: - the tracks are encoded with Dolby atmos that allows encoding positional data for sources. These don’t match up to channels on an audio system in anyway because it gives them flexibility - the audio apps simply provide the atmos encoded stream which is pulled into the cars own audio system. The audio app needs no knowledge of the target playback device. - the cars audio system knows where the speakers are so can then playback sounds on each relative to where the atmos sources are positioned. - this isn’t done over Bluetooth. In this case the Apple Music app is running natively on the cars MBUC system. If it weren’t native, it would use CarPlay which isn’t using Bluetooth for either its wired or wireless mode.