Hello CeltGirl,
It sounds like when you are in a car, all devices go into driving mode, but you want just the one connected to Bluetooth.
Here are some resources that can help.
Review Use the Driving Focus on your iPhone to concentrate on the road and Stay focused while driving with iPhone
- You can set your Driving Focus to activate When Connected to Car Bluetooth instead of Automatically which detects "When iPhone detects you might be driving."
Furthermore, if you are sharing the same Apple ID across devices, check for the Share Across Devices setting as seen in Use Focus on your iPhone, iPad, or iPod touch.
Hope this information helps.
Have a good day.