Is it possible to open and use a different app on iPhone while using CarPlay or is the CarPlay only mirroring the iPhone? More specifically would it be possible to have Waze running on iPhone and use different apps such as music apps on CarPlay simultaneously? If not what does happen when some non-CarPlay app is opened on iPhone? Does the CarPlay go to home screen?
Essentially, as soon as you use any other app, CarPlay will drop to its home screen.
You can have Waze running backgrounded on the phone itself, so you can hear its announcements, but you cannot enable it on the CarPlay screen.
Whilst doing this, you can return CarPlay to any of its own apps, music, map etc, or any of the specific apps provided by the CarPlay manufacturer - e.g. Pioneer [except for a bug in their AppRadio mode which breaks under iOS 11 making it totally useless.]
There's a video on the Pioneer site showing Waze working in AppMode... which I've never been able to achieve. It also shows 2-finger zoom like an iPhone, which my CarPlay cannot do, so maybe it's a newer model than my SPH-DA120.
This may all come down to the make & model of your CarPlay system, unfortunately.
If it's Pioneer, you won't get it working, until/unless they fix the AppMode bug .