Ah, I think I understand what you're getting at. The core concept I think you're missing is that there is no "car computer". There isn't a single central computer running everything, with just wires to remote sensors and actuators. Instead, the car is built from of dozens of distributed systems, all interconnected via the CAN bus. The engine has its own computer, and its tasks include firing spark plugs, monitoring engine sensors, etc. The ABS has its own computer, and all it does is to monitor and modulate the brakes. The dashboard is its own computer, and simply displays data coming from the various other systems. And the infotainment system is its own computer, and is hideously complex, and does no end of crap. I believe my car has over 140 individual systems on the bus.
Each device on the bus has an independent processor that does whatever the device is supposed to do, a CAN controller, and a transceiver that isolates the device from the bus (thus preventing a wonky stereo from shutting down the engine.) Some devices transmit data constantly, such as the engine controller continuously sending RPM and exhaust gas temperature. Some devices only transmit data when they do something "interesting", like the seat belt detectors or the tire pressure sensors. Others don't normally transmit much data, but instead read from other sources of data and do something with it. The remote control mirrors are an example of a device that doesn't send much of anything, but constantly listens for other information.
Each system works independently, talking to and from the bus as required. The bus protocol arbitrates amongst itself to figure out who is sending the highest priority message, so things like airbag deployment can take precedence over changing the radio station. The CAN bus is that standard interface you were asking about.
The "infotainment" system has expanded beyond sound and is becoming the center of control for things like climate and navigation. When you tap the "A/C" icon on the screen to turn on the air conditioning, it'll send an "calling for cabin A/C" message over the bus. But it's not in charge of your car. Your car will continue to work even if someone pries it out of the dashboard, (as long as they didn't steal the security system, too.) You maybe won't be able to adjust the climate, you won't have a satnav system, but your car will still work. And you could replace that panel with a different panel. The horrible "MyTouch" control panel is an option in Ford cars (unfortunately standard on their higher end packages), but you can get simpler option packages that don't include it, and instead have a panel with actual tactile controls. Both panels use the same CAN bus and interface to talk to the rest of the car's systems.