It is possible to make your own! You can use an old tablet, or even better a raspberry pi and touch screen. Then you add a hat with a better dac and send the audio to an amp. It’s really cool, but I’m pretty sure just buying an actual aftermarket unit will get me a better unit for less money and much less work…
True, I was never going to do that though so it didn’t matter for me. I just want a receiver for Android Auto and my phone will do all the connecting and thinking.