How can i select other apps when running Android Auto

[AC Question]

When I connect my Nexus 6p to the car Android auto automatically starts and on my phone screen I can just see the text: Android Auto and I can't do anything with the phone.
My wife connects with HTC M9 to the car and she can see on her phone all Android Auto compatible apps and select any app she wants. How can I do it on my phone? Currently if I want to play youtube for example I can't since Spotify is my default music app