Do all vehicles with OBD2 have a CAN-bus? Otherwise, what are the alternative buses used?
Are there special versions of the buses for trucks and school-buses than for cars/suv/vans?
The answer would be yes if the vehicle was produced after 2008, otherwise no. in the following article it states it is required as one of the signalling protocols in OBD2 Post 2008.
So presently vehicles that are currently being produced do have a CAN channel included in the OBD2 Connector. However, there are are pins in the OBD2 that are manufacturer specific. And there are usually multiple CAN-buses in the modern Auto-mobile, as I have drawn from my experience.
You will also see the alternative buses in the link as well.
Again, please be aware that much of OBD-II is manufacturer specific.