How do I connect flash drive to iPad?

Connection of external storage devices is possible - with some caveats...

To be recognised by iPadOS, the attached USB storage device must be formatted to FAT, FAT32, exFAT, APFS or HFS+ with a single partition. As the iPad has limited power capabilities, dependent upon the Storage Device, an external power source is often required. For HDD, an external power source is definitely required.

If your iPad Pro has a Lightning port, you will need to use an Apple Lightning to USB3 Camera Adapter to connect USB devices to the iPad. This adapter has both a USB 3.0 Type-A connector and an additional Lightning port to supply power, from your Power Adapter, to both the iPad and connected device. The Power Adapter must be rated at 12W or more.

• Lightning to USB 3 Camera Adapter

https://store.apple.com/uk/xc/product/MK0W2ZM/A//:0" height="316" width="94

Some additional information about connecting to external storage devices can be found here:

Connect external devices or servers with Files on iPhone - Apple Support

Look into the Apple USB 3 Camera Adapter.

You’ll use the Files app to access the files.