I'm using 'Plugable' USB BT4 adapter for connecting audio and calls to Open Auto Pro, and a UGreen USB sound card then plays the audio out to my car aux in. The USB Sound card also has a mic input, but I cannot work out how to get the mic working at all for any 'OK Google' or as the mic for phone calls.
The mic is showing as an 'Input Device' in the PI Volume Control app (as 'USB Audio Device Analog Mono').