For what it’s worth I’ve heard they’re basically identical. It’s just some two-way data and an h264 video stream. There’s a reason basically everyone supports both. Seems to be really easy.

I don’t know whether the auto makers actually forced them to be compatible or it was a choice in Google’s part to get into cars that already had CarPlay.

But it really doesn’t seem like it’s a big hassle. Most of it is probably just certification testing to be allowed to use the names/logos.

My car has wired carplay/android auto, but I have a nifty little USB dongle that receives carplay/android auto wirelessly and passes it to the car via USB. Sometimes called an AI box, I think.

These things contain a whole System-on-a-Chip board. I presume what it's acting like a middle box; pretending to be the headunit to the phone, and then sending the phone's output to the headunit, pretending to be a phone.

Since they need a quite beefy CPU to do that, I'm guessing they don't just pass along packets, but actually speak the protocol on both ends, and perhaps transcode the a/v stream.