The passenger can grab the phone that is connected via BT, CarPlay or Android Auto. In the case of CarPlay, someone in the backseat can connect to the phone that is using CarPlay by using Shareplay and control the music.
Well two things - if the driver has a CarPlay connected and then used SharePlay to allow the other person to control the playlist, then the other person can play anything they want to.
In our 2025 Kona - one of the cheapest cars sold by Hyundai - you can have CarPlay connected with one device and have another phone paired with BT for audio.
CarPlay doesn’t use Bluetooth. It is either wired or using WiFi direct
A passenger can’t use your phone connected with BT and control the audio?
But it looks like the USB port in that model year Subaru supports the iPod protocol meaning if you have an iPhone, why wouldn’t the passenger be able to control the music?
If you have an Android it looks like it supports just using your phone as a dumb mass storage device that contains music.