@Wolksby @andrewilley from Poweramp point of view, there are no wireless, BT, USB, or wired connections. There is just single audio "sink" (native AudioTrack API or AAudio) which is used (except Chromecast output, which is non managed by the system).
Android routes audio signal internally as it seems fit, this routing is hidden from the app, and it's even hard to get the actual output info and its features, as routing info is often buggy, spreaded over a few different APIs, and/or very firmware/Android dependent (though, it's now better on the recent Androids). Poweramp has no permissions to change audio route - this is system level/root only permission. There are few hacks which can be used to e.g. force switch speaker off (Force Speaker Off for Hi-Res Output option), but nothing more.
On the recent Androids and on Samsung firmwares the routing can be changed via the media notification:
Also, Android Auto UI and audio is rendered on the phone and then compressed either to video + bt audio or to video + audio in one stream. Audio is usually lossy re-compressed in both cases, though there is no public documentation/specification on this and it may vary across devices/firmwares/headunits.