First, it is certainly possible to connect a USB thumb drive (or other USB storage device) to your iPad9. To do so you will require an Apple Lightning to USB 3 Camera Adapter:
https://store.apple.com/xc/product/MK0W2ZM/A

Don't be fooled by the name of this Adapter - as this Adapter contains the essential USB Root Controller that is required to connect USB devices to an iOS/iPadOS devices that feature a Lightning port.
To provide sufficient power for the USB drive, you’ll need to connect a Power Adapter (charger) rated at least 12W to the auxiliary Lightning port of the Camera Adapter. The iPad, on its own, except for very low power devices, cannot supply adequate power for connected USB storage devices. The Power Adapter supplied with your iPad, with the appropriate cable, is likely to be adequate.
In all cases, your USB storage must have a single data partition - and be formatted as FAT32, exFAT or APFS. iPad cannot read multi-partition devices, or proprietary device formats (such as NTFS).
Connect external storage devices to iPad - Apple Support
Now to the second element of your question. Many native App Apps require files and photos to be imported from USB storage to the iPad before they can be accessed or edited. Some third-party Apps are coded to both access and edit "in-place".