> …since I'm an Apple Music user, whenever I start the car, it just defaults to whatever's I've been listening to there… […] Now why apple just defaults to Apple Music, and not "your music player of choice a.k.a. Spotify" is questionable…

As you noted, it continues playing whatever you were playing in whatever app you were using, with no preference for Apple Music (I'm also a Music subscriber). If you were playing something with Spotify, that's what will continue to play.

I do wish it were configurable with contextual defaults. Too often, I start my car and my sleep sounds app continues to play "Airplane Interior".