Same problem with my S20 (with Note 9 earlier). I have a sony player in my 2005 model car, only USB and Aux are my options.
My wife's iphone XR just works fine.
I had a Bluetooth to Aux converter from wish.com for $1 and it worked well until it was broken. Probably that's the only option for me (about $7 in local retailers). Or to use the type c to 3.5mm converter.
When I allow USB read on my phone it takes few mins and plays some random audio. Later I figured out it was playing audio files from the whatsapp audio folder. It plays these files irrespective of what is playing on my mobile, very weird.
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