Trigger seems to be iOS thinking it's being helpful so that when I "plug" headphones (either physically, bluetoothically, or carplayly) I presumably want to play music.
Also, double click on Library in Music.app nee iTunes on macOS. Plays the whole library, linearly, which, like, who does that?
Often, when I see a feature like that, one that makes you ask "who would want to do that with the whole dataset?" — the answer is usually "developers regression-testing their feature branch of the program, where their 'whole dataset' is a test fixture consisting of a bunch of data samples where each one exercises a weird edge-case path in the code."
Sure, you could just make a playlist for this. But iTunes has a directory it watches within the library, where putting stuff in it will cause iTunes to automatically move those songs into your library. And if I were an iTunes dev, my scripted "Test" action would consist of creating a new library directory structure; plonking a copy of my regtest fixture dataset into its auto-import dir; starting up the new build targeting that dir; and then sending it the Automator action "Library → Play All." A playlist would only complicate that.
There’s gotta be something else going on because I’ve never had this happen over the last decade. I’m pretty sure the car is sending a play command to the phone every time it connects.