In my personal experience most original manufacturer systems listed above are tied together by a network of some sort, specially in higher end vehicles.
Most US vehicles use CAN, but there many more like Byteflight, FlexRay, LIN, MOST, and even I2C, etc.. There are a bunch of ISO standards, and even open source solution for such networks, including OS (like OSEK).
The OBD interface is like trying to enjoy a digital forensics amusement park through a pin hole.
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