Somewhat related, that's how guns are classified in the US: The Gun is "just" the lower receiver. Even though that one is useless by itself, it is the foundational piece of it to which one attaches all the accessories. (And even then there's loopholes, like how a 80% lower receiver is just a piece of metal and not a gun yet)
With a car, I guess one answer would be "The part that has the VIN on it is the car", but apparently there's no universally agreed on location. I would probably go for the drive shaft.
“Lower” and “upper” receivers don’t apply to all guns, just certain types that were designed that way, e.g. the AR-15. I’d estimate more guns don’t have separate/distinct receivers (if the term even applies to them in the first place)
With a car, I guess one answer would be "The part that has the VIN on it is the car", but apparently there's no universally agreed on location. I would probably go for the drive shaft.