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You can add YouTube, web browsing, and more to your car by sideloading apps.
Android Auto does a lot of things well, but it's a little limited. Scrolling through the Android Auto section of the Google Play Store, I find a decent, but not huge, catalog of apps. Google places hard limits on what it wants your car's screen to do, yet Android Auto is capable of so much more.
This is where sideloading comes in, or installing unofficial apps from sources outside of the official Play Store. These apps significantly expand what Android Auto can do (you can sideload apps to your phone, too), and the process isn't difficult.