I won’t claim to be a CarPlay expert, but it is my understanding that it just mirrors your device on the infotainment system of the vehicle.
Yes, it's basically just an I/O device. When your phone is disconnected from it, there's no data left in the car except whatever is needed for wireless pairing (if you use wireless pairing) in the future. With wired CarPlay, it retains as much as a dumb touch screen display would.
Additionally, even if Toyota were to get breached, they would not get my data