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.