Ok to clear some things up.
Currently there are 3 kinds of carplay scenarios.
1. Carplay second display mode. Currently Apple only. Custom interface created by the phone customized for the app. This is how all the apple first party apps work with the exception of the podcasts app.
2. Carplay media player apps. These send data to some kind of carplay music app that the phone has. The interface for all these apps is the same. The best way to understand this is that it is a kind of media remote control for your open app. It can read metadata from the app, scan playlists, artists saved, whatever, and the developer can create custom buttons for the app media control app if they wish, like in iHeartRadio you have a like this song, and a bookmark button. In the podcasts app you can go back and forth by 30 seconds.
3. non supported app control. This only allows for play/pause and fast forward/rewind. Any non supported media app of any kind uses this. No custom API required.
Few other things.
1. Any sound coming from your phone will go through carplay.
2. Your phone is not locked out while using carplay. So that means...
3. You can run ANY unsupported app while in carplay, including waze and google maps, but they will not display on the headunit. They will however send their audio through the car speakers, use the car microphone, and display on the iPhone if they are the active app on screen.
4. Apple has not yet opened the APIs to allow more powerful apps to be used in carplay, but when the original iPhone launched the only apps they ever allowed were Web apps until they had a good plan forward. So I doubt this situation is forever.
5. Any audio source can be used at the same time as carplay that is supported on the iPhone with only one exception and that is bluetooth audio. It is disabled on the Pioneer while in carplay. However, the USB 2 can accept another iPhone or android device, the HDMI port works, as does the aux or any other port, or the HD radio or the Satellite radio if you have that module for your Pioneer.
6. To do this simply press the home button on the Pioneer, or the virtual one in carplay and then the Pioneer button on the display, and then from the source menu click the audio source you want to listen to. You can also go back to carplay to use apple maps check your messages or take phone calls. The Pioneer radio will happily mix all sources together. You can even hot swap between the control interface and Carplay by pressing the mode button.
7. This also works in Reverse. If you have a NAV unit (5000-8100) you can run the built in Nav while using carplay to control your phone music and whatnot. Again the headunit happily mixes all sources.
Hope all this info Helps!