Because Google is great at naming things, you might be getting confused between a few distinct products:

• Android Auto lets you tether your phone to your head unit (via USB cable, some units support wireless connections as well), and access cut down versions of some apps on your phone via the head unit - it's pretty common on first party units on late model cars, and has the advantage that you don't need a data plan (it uses your phone) and don't need to keep your phone and your car in sync - everything is running on your phone directly. Apple has an equivalent called CarPlay
• Android Automotive is a special version of Android designed to run directly on head units - it's licensed to OEMs and usually skinned to match the manufacturers branding. I've never seen Android Automotive in the wild (it's only available on first party units for some specific model cars), and I'm not really clear if you can install arbitrary Android apps or not
• Some after-market head units come with Android installed (they are essentially tablets in a head unit form factor with the appropriate I/O connections on the back) - depending on how professional the manufacturer is, they might brand this as "Android Auto", even though it has nothing to do with real Android Auto. These typically will never see an update, and probably don't meet the IP as specifications they claim. You can typically install whatever apps you want, but make sure you understand your local laws, where I live having a screen playing video "within eye line of the driver" will get you a fine and points on your license, so playing YouTube while driving is probably a great way to have an awkward conversation with highway patrol