I was actually handed a Volvo brochure by the salesman showing Pandora as an available app, but as you say, it's not available. Some deal must have fallen through. Spotify is, after all, a Swedish company. I used Poweramp to create playlists on my phone and using bluetooth to play. It works okay, but it was a royal pain converting all my WMA files to mp3 and copying them onto my Android phone, then creating the playlists. It took hours. I had to use a stylus to check all the little boxes or select options.
The only reason there is no Pandora app is because Pandora hasn't developed and released an android automotive version. There is no restriction who can release an audio app for android automotive that will show up in the play store. Where it gets trickier is things like A Better Route Planner on the Polestar 2. Google is not allowing non-media apps right now so Polestar worked with A Better Route Planner to release their app with special permission.
The best hope for apps like Pandora is that more cars get Android Automotive and they finally decide to support it. Some of the big auto groups are switching to it now in the new vehicles so I believe the app store will grow in the next 1-2 years and hopefully google will open up different app categories such as navigation and video.