It is quite strange that an app (such as Pokemon Go), can output sound only to the car by bluetooth. If it is the "iPod" (using a lightning USB cable) to the car, then songs played by the Music app will come out as usual, but the Pokemon Go audio will not.
Also, if I turn off the car and walk far away from the car, the iPhone cannot output the Pokemon Go audio by the speakers. The Music app can output a song as usual.
The only way is to attach a headphone and you can hear both.
This is true for iPhone 6S Plus and iPhone 7 Plus.
I know this may be related to how an app is developed, but I thought the audio output channel selection is more of an operating system thing (iOS). So if we output the sound by bluetooth, then everything should be by bluetooth. It will be strange if each app is tied to different channels -- Angry Birds by bluetooth, and Cut the Rope by speaker.
I guess more importantly, how to make it output back to the speakers or by the lightning cable? I tried turning off bluetooth on the iPhone and it is no use.