OK, this is how streaming etc. work with my iPhone (6) and my car (gx4hs) - hope it is helpful.
If plug my phone into a usb charger/don't plug it in at all, when you turn on the car it tries to leave off where it last was: so if you were playing spotify/audible/whatever, it tries to continue playing that. I didn't check if this works if the app is fully closed (i.e. not even in the background).
If I plug my phone into the usb port in the centre console, all bets are off! The car detects an iPod and switches audio to iPod mode and appears to be playing music from the music app, however it is actually playing the music through bluetooth - because I couldn't hear anything until I switched audio to bluetooth, at which point I heard the music playing. Also, if I try to start playing an app over bluetooth the car may switch back to iPod mode and it sometimes seems to get confused between iPod mode and bluetooth streaming.
So, for my sanity I'm simply going to plug my phone into a usb charger in the 12v accessory socket in the armrest. One day if the software gets updated and plays nice maybe I'll switch the cable over to the built in usb socket...but not today.