1) Bluetooth sucks. It's a protocol designed to transmit Danish business cards. Even 20-odd years later it's an inferior audio protocol.

2) Under Bluetooth settings, per device, you can set whether or not the phone goes there.

3) Connecting your phone to your car's bluetooth sucks. Don't buy a car that lacks Android Auto & Carplay or lacks a DIN or double-DIN stereo. My wife's Fit fails all these tests but it also has an audio in jack.

4) That said, notifications can be silenced. It involves a lot of fiddle-fucking in the preferences to get it to work but in general, flipping the STFU switch on the side of an iPhone radically improves life.

5) on an iPhone call screen, pressing the "source" button will swap the call to wherever you want it to go. In a car that shouldn't be AirPods. It's a moving violation.

6) Your phone's operating system does have a mixer. It also has a matrix router. However, that mixer/router doesn't have a handy-dandy GUI, it has a bunch of if/then statements and individual volume controls. And yes - Bluetooth is only one destination, and that is appropriate because Bluetooth sucks balls.

These are all surmountable problems. You just have to start from the standpoint of "I can't be the only person this bugs the shit out of. Surely other people have addressed this ridiculously annoying issue." To be fair to device manufacturers, most every device manufacturer has gone "the less control we give to car manufacturers the better." Carplay/Android Auto are vastly better experiences than anything any car company or stereo manufacturer can come up with. They're imperfect? But substantially more livable than Chevy.