This is my understanding as to how it works. I copied this from the iPad iOS 13 iBooks User guide.
“Connect a USB drive or an SD card
Insert a USB camera adapter or an SD card reader into the charging port on iPad.
Depending on your model, use the Lightning to USB Camera Adapter, the USB-C to SD Card Camera Reader, or the Lightning to SD Card Camera Reader (sold separately).

Do one of the following:
Connect a USB drive: Use the USB cable that came with the USB drive to connect the drive to the camera adapter.
Insert an SD memory card into the card reader: Don’t force the card into the slot on the reader; it fits only one way.
To view the contents of the device, tap Browse at the bottom of the screen, then tap the name of the device below Locations. If you don’t see Locations, tap Browse again at the bottom of the screen.”
Excerpt From
iPad User Guide for iPadOS 13.1
Apple Inc.
https://books.apple.com/us/book/ipad-user-guide-for-ipados-13-1/id1464778568
This material may be protected by copyright.
This is an adapter that Apple sells for that.
Lightning to USB 3 Camera Adapter
https://store.apple.com/xc/product/MK0W2AM/A
I made type bold for emphasis.