I concur this is confusing. There are 3 ways to “copy” cards:
copy to clipboard, which you can then paste in the same space or you can navigate to another space and paste. This is done using keyboard shortcuts. btw, this doesn’t work across tabs or browsers.
copy to another space (through the menus).
copy card names (through the card dialog). This puts the card contents/names into your clipboard, primarily to use in another application.
You can also copy the card names of all the cards in the space through the space dialog, Export.
I don’t have any suggestions at the moment how to improve this, but wanted to point these out. I tend to use the “copy cards to space” function the most.