The CAN bus is a twisted pair network. It connects the various modules together.
https://en.m.wikipedia.org/wiki/CAN_bus
For example, when you push the drivers side window down button, you are activating an input in one of the SAM (signal acquisition module) behind the dash I believe. You are actually switching a resistor on a sensing line. Each button has different resistor values so the SAM looks at the sensing line and based on the voltage on the line determines the function desired.
The SAM puts out a message on the body CAN bus that the driver window should go down. The door module sits on the body CAN bus and picks the "driver window down" request out of the stream of commands going by on the bus. The motor gets switched to drive down the glass.
There are two buses on the 210, body and power train.
You had an encounter with the power train CAN bus with the issue you had with the glow plug relay a while back. 210 glow plug relay is not like a 123/124/126 where it gets a signal from the key switch, on the 210 a CAN bus message commands the glow plug relay on and then monitors the status.