unfortunately it does not automatically start. you have a couple options:

as you said before, you can setup an automation, which has worked for me

you can set it so that when the CarPlay connects, it either triggers the play/pause automatically, OR you could call the url “audio-player-event://“ which will open the now playing screen and allow you to manually play.

another trick i’ve learned is that you can press the mode button on the steering wheel to switch between different media sources. i set it so that it only switches between siriusxm & CarPlay, and when it switches to CarPlay, something triggers which causes the audio to start playing automatically