Good afternoon mak209,
Welcome to the Apple Support Communities!
It sounds like your iPhone 7 is not staying connected to your 2020 BMW.
Check to make sure the iPhone 7 is updated to the most current iOS.
Then, make sure your vehicle's firmware is updated as well.
Try forgetting the iPhone 7 in Bluetooth settings on the vehicle, then restart the iPhone and vehicle and setup again.
The resources below will help you with this.
Update your iPhone, iPad, or iPod touch
How to connect your iPhone, iPad, or iPod touch to your car
If the issue persists, please reply to this thread and we'll do our best to help you!
Happy Holidays!