Don’t take my word for it. Tear into any one of the dozens of black boxes in your car and take it apart. Analyze the chips soldered on the boards. You might get lucky and find all standard chips with information available from suppliers.
Try looking at the data going across any one of the several buses transiting your vehicle. OBD is easy. The others are usually encrypted and much higher speed.
Cars are legitimately complex. Don’t just listen to the manufacturers and scoff. Look up some research into breaking the communication protocols that MB or BMW use. Compare that with GM’s newest standard. Go ahead and practice your reverse engineering skills, because these things aren’t published.