Android Auto has a new Swipe To unlock feature that allows phone interactivity, which is likely to be a huge benefit to some users with Android Auto in their vehicles. Google has yet to confirm the addition of this feature and it's not clear when exactly it was added, but it had to have been recently as it wasn't showing up for some users within the last few weeks.
With no announcement from Google as to the reasoning for bringing this feature to the platform, it's also unclear what Google is aiming to do by adding this in the first place. Initially Google disabled any use of the phone while it was plugged into the car for use with Android Auto so people wouldn't be able to use their phones while driving. This was Google's effort to help stem the tide as it were in regards to vehicle accidents and the potential danger people could put themselves and others in by using their phone behind the wheel. Now, though, Google seems keen on making the phone accessible, for whatever reason.
If you have Android Auto in your vehicle, all you need to do is plug the device into the system to power the in-dash display, and then when you go to unlock your phone's screen you should see the new swipe to unlock option. Upon swiping up you'll get access to the regular home screen instead of just the "Android Auto" screen. If for any reason you need to access something from the main home screen of your device while it's plugged into Android Auto, as soon as you lock the screen you'll need to swipe up to unlock again every time thereafter. It's not clear if this is available to all Android Auto users or if this is just starting to roll out to users more widely, but chances are it'll be a welcomed change for some.