When I plug it into the car, it automatically starts playing the same song on Apple Music
I want it to start playing Spotify instead, but that can’t be configured
Either I turn off CarPlay altogether, or I let it do what it wants
Well behaved radios don’t send that signal immediately, and instead wait for you to hit play on your device, or on your cars radio/stalk.
My car (maybe most?) helpfully sends a "play" command when my phone connects via Bluetooth. iOS devices play what was last playing, or pass the event to your music app if you've played something else in the meantime.
The fix in my case was a Shorcuts automation that gets triggered when the phone connects to your car. I set the volume to max and tell my favorite podcasts app to "Play".
If I just wanted an app to always be considered the "last played" app, I'd do something like add a one second delay and then pause. If I used Spotify, I'd also delete Apple Music.
I even trained siri to use tidal by saying “play artist on tidal” a few times. (Siri is completely useless for this use case, but it does not open apple music.)