I think I got it.
On my Samsung S10 I did the following
Settings - Apps - Samsung Music Settings - Music w/ Screen Off
This worked to prevent autoplay with my Ford Edge with Sync. BUT, If the very last app I used was a music app or pandora, spotify etc AND it was connected to the car when last shut off, then it will come on again. If the app was not connected to the car when last used it would stay off when starting the car.
I use Google Play for music and this is an issue for me. I have researched settings within the car, Bluetooth, and Google Play app to no avail. The car and phone autoplay music from the Google Play app once the audio on the radio is changed to aux. The above recommendation would prevent music from playing somewhere like at the gym if the screen is off. This is frustrating because i may drive home on a phone call with the volume up and then start the car to leave, the phone starts playing music from Google play automatically with the volume up. It is annoying, so because I have a Samsung i have to remember to turn the volume down when in aux mode