How do I connect a USB drive or an SD card to my iPad to transfer files?

You can use the Files app to access files stored on external devices or servers, such as USB drives and SD cards, file servers, and other cloud storage providers.

See this support article in the iPad User Guide:

https://support.apple.com/guide/ipad/external-storage-devices-ipad75b7b23f/16.0/ipados/16.0