Does Android Auto use USB or Bluetooth audio routing for UAPP? How can I check?

[Skelenton]

I play music in my 22 Subaru with Android Auto over USB (A to C), but I cannot figure out how to check if my music is playing over bluetooth or actual USB. Android Auto dies as soon as a turn my phone's bluetooth off so I assume it's bluetooth, but is there a way to check? Or even better, can I get UAPP running over USB onto my car with/without Android Auto?

Very stupid question I know but if it's going to just play over bluetooth anyway then I can stop plugging my phone in and dealing with the buggy mess that is Android Auto altogether.