Thank you both for your replies!
I don't really understand what you are saying... I have a fairly new Volkswagen with a built-in head unit which features Android Auto. I can connect my cell phone (Pixel 2) via USB cable and the head unit shows Google Maps, Waze, Poweramp, Spotify and a bunch of other apps. I know there are head units which run Android natively, This is not what I'm talking about.
So if I understand you right, the UI using Android Auto (shown on the head unit but needs a phone connected) cannot be customized?
If I got this right your skin will also not change the UI on the head unit, right?