Functionally, there’s actually more overlap, but there’s of course a huge difference in that Android Automative is an operating system, where, quite simply, Android Auto and CarPlay are just a distributed interface or overlay. The entire job is done by phone.

The biggest advantage of CarPlay and Android Auto is that you’re always up to date. Historically, car manufacturers have not been very generous in providing long-term support for their multimedia systems in the form of feature updates. (At most some bug fixes if you’re lucky).
I don’t know anything about Android Automotive. Can this be updated from Google? Or should this also be done through the manufacturer, because you still have the same problem in that aspect.

Of course, Android Automotive pretends to be more than just a multimedia system because it also has some components of the car in the same interface and can make some connections between them, but the degree of “real integration” and logic often remains undefined visible.
Anecdote: My car thinks the driver is the only one who wants to listen to the radio. When I turn off my engine to refuel and open the door, the radio turns off completely. Regardless of whether the seat and belt sensors detect the presence of other people in the car. These people then have to turn the radio back on themselves.
However, if someone else goes to refuel and the driver’s door remains closed, the radio will still be on.

Subsequently. The “non” multimedia system is fully integrated into the car. At most, it shares one interface, but there is no real integration.