Its clear that the physical and data link is covered by CAN. But does all of the upper layers covered by CANopen? Should it cover all? What confuses me more that there are sewveral CiA documents (such as CiA DS 301, 303-1 ...). Which document(s) describes the protocoll called CANopen?
The mid layers (3 to 6) of the OSI model mostly apply to the various Internet protocols and aren't very meaningful to apply in the context of CAN buses.
The most important part to understand is that CAN is only the 2 bottom layers and not some stand-alone communications product. "We must use CAN as our communication protocol" is thus a nonsense requirement unless also specifying which application layer to use.
For the most part, CANopen is an applications layer.
Some parts of CANopen could be said to belong to the lower layers, like for example segmented SDO, which is used as a carrier protocol, belonging to the transport layer. Similarly, automatic re-transmission as mandated by the CAN hardware could be said to belong on the transport layer as well.
More info can be found here: https://www.can-cia.org/can-knowledge/