Android Auto works with out a WiFi or data connection On my mobile phone, I have the WiFi and data connection disabled.  Yet, I still get  google maps via my Android Auto as seen on vehicle display.  How is this possible?
For me at least, my local area has been automatically downloaded to be used offline. Has yours?

In "normal" Google Maps, outside of Android Auto, go to the hamburger menu, then Offline maps. Mine's currently set to "Home, 52MB, expires 21 April 2020", which corresponds to a square about five miles across.

There's loads of settings in there for you to tweak.