Old service manuals used to explain things much more than today. The manual of the Shugart SA410, shown in my previous post:

 

These, I think are more interesting:

https://www.plrelectronics.com/xcar...usb-drive-slim-laptop-size.html?category_id=9

They convert the floppy interface signals into a "Floppy Drive". It seems like nothing is standard.

In the Floppy world I dealt with 8 1/2" for the PDP-11/2, 5 1/4 for the IBM PC, 3.5" drives for the MAC, PC and an HP-85/86 computer that used an IEEE-488 cable to the floppy box.

We wrote a kermit program to get the files from the HP 85 machine.

and this gives you an idea of the complexity of maybe we thought was trivial:

https://www.plrelectronics.com/lab/usb_floppy_emulator_technical_details_and_instructions.php