I used to design CANBUS in cars for a living, The easiest way to understand it would be to think of a twitter network for your car, with each module having an account to shout things out.
The headlamps will be controlled by the BCM (body control module) or they might have a separate Headlamp control module (HCM).
This will supply power to the lights and monitor what current they use to ascertain if it is healthy or not. On start up it will just pulse a signal to confirm the loop is connected, IE just make sure a bulb is there. When the lights are on and stable, it will monitor current going through each bulb and if it detects the current draw is out of normal range it will flash up a warning.
There is no such thing as a CANbus bulb etc or anything like that, all that happens is LEDs use a lot less current than normal filament bulbs, so to the module who is looking for a normal resistance from a filament bulb it thinks it has short circuited and is now faulty as very little current is drawn and so sends a message saying Cluster please light up the bulb warning light.
The error free bulbs just have resistors stuck on them to boost the current drawn and trick the Module into thinking the bulbs are good.
the cluster sees this message and does so.
CAN has been going on most cars since pre 2005 and is now industry standard although there are a few other networks in your car such as LIN nodes and sometimes Ethernet/fiber optic.
If you have any questions ill be happy to answer them.