(Windows 11 Pro, version 23H2 - on a laptop computer)
FIXED!
My newly-purchased USB floppy disk drive does not appear in Windows Explorer BUT, if you type A: in the search bar, there it is!
In Device Manager look under the heading "Floppy Disk drives" to see the automatically-inserted driver for your FDD.
If the driver exists there, then Windows found your legacy drive and installed a driver. However, since nobody uses drive A: and B: any more, they seem to have seen fit to remove any reference to them in Windows Explorer, and you have to go looking for yourself.
Cool.