It usually takes a few months after new camera introduction (Fall 2020) for the camera vendor to provide Apple with the information necessary to support their newest cameras, and then internally, for Apple to test, and schedule the camera RAW support updates in a future operating system update. The earliest I would expect the Z6 II and Z7 II would be Big Sur 11.2, and that might be optimistic, without knowing Nikon's or Apple's internal schedules.
Here is the living Big Sur camera RAW support document, and when the Z6 II and Z7 II appear, that will signal their Big Sur support.