iPad lighting USB adapter

Well, I did answer your question in confirming that you can connect (i.e., is compatible with) your iPad9 - and with it connect USB storage and other devices to the iPad.

Many native iPad Apps require that you first transfer photos and files to your iPad for viewing, but some third-party Apps van view files directly from connected storage devices. Faster storage devices will obviously work better than those that are slower - both in reading from and writing files to the attached storage.

Presumably you refer to this Lightning to USB 3 Camera Adapter?

https://store.apple.com/uk/xc/product/MK0W2ZM/A

Using this adapter, you can connect USB Storage devices to either your iPad9 or your iPhone. You can also connect some other USB devices, such as a camera, wired keyboard or mouse.

If used to interconnect your i{ad and iPhone, you’ll not be able to directly access or transfer data or files between devices - except when performing a full data transfer when initially setting-up a new device. The wired interconnect can also be used to share the iPhone hotspot, with the iPad, over a wired USB connection.