Far and away my least favorite aspect of Android auto is not having a simple, easy option to toggle auto connect.
Another poster shared how to disable auto connect through settings - when I went that route my phone no longer would connect to the car. I had to do it,, however because it would auto activate my Bluetooth and fight my wife's phone to connect in our car.
Android auto started as a phenomenal idea, but through its quest to be as convenient as possible, it went too far and became incredibly burdensome if you have multiusers or don't want to always connect.
Sorry for my rant, Android auto has really been a frustration for me recently, enough to the point where I've uninstalled it entirely and am back to using my phone on a mount.