Craig Federighi clearly said multiple times in BOLD that Apple will never merge macOS & iPadOS. I think the same goes for hybrid devices.
The funny thing is, Steve Jobs denied that Apple would ever develop a phone, too... until it was time to actually release the product. Then he (and Apple) made history.
For the transformable crowd, how does the switch from iPadOS to macOS happen?
Noting that others have already addressed this in part... I would only add that a "hybrid" OS wouldn't necessarily have to "switch phases" at all, nor does it have to be fully either one or the other; it can be an entirely different beast all by itself -- and even as such, it could still run both iPadOS and macOS software. Why not? MacOS already does.
Public declarations to the contrary aside, the groundwork for a hybrid/merged OS has already been laid. Thus, I think this is what we're really all talking about:
foldOS
You heard it here first, folks.