Thank you for your response. Unfortunately, none of your proposed solutions did the trick - turning "Handoff" in "AirPlay & Continuity" (which is what it is called in iOS 26.1) off did nothing to stop the autoplaying of songs from my iPhone as soon as I turn my car on. Your "nuclear optIon" didn't work, either - in "Allowed Apps & Features", "Music" isn't even listed, and turning off CarPlay doesn't seem to do anything. (As an aside, "CarPlay" is really an odd app - it is nowhere to be found on my home screen - even when I search for it - and it doesn't seem to have its own "Settings" - but it does get listed among the "Allowed Apps & Features" list and in other places).
My issue in any case is not so much "Handoff" or "Continuity" - I am entering my car with the Music app closed and nothing playing - yet as soon as the car recognizes the iPhone, it somehow triggers Music to start up, select a (more or less) random song and begin playing it, even if I haven't listened to that song in a long while. It may be more of an idiosyncratic behavior by my Subaru, so I guess for now I'll just keep the volume down and once I need to interact with my iPhone (Maps, read-aloud messages, hands-free conversations) I can manually stop whatever is playing in Music and turn the volume back up. Unsatisfying but workable...