Copying kindle books from mac

[StuartC]

I have asked this in another forum but after as many insights as I can get! I have a new Kindle paperwhite, my intention was to use this as a standalone reader not registered with Amazon. I copied kindle books from my wife’s paperwhite to a folder on my iMac. As I understood it I thought I would be able to copy them directly from the mac to the new kindle as they were in a kindle format, in this case kfx. I can do that but the new kindle says my library is empty!
Calibre does not seem to be able to help even with deDRM plugin.
Clearly, either I have misunderstood or am missing a step. Why will they not seamlessly transfer? Is DRM blocking this transfer?

Any insights very welcome