Importing photos from usb flash drive

Do you have USB-c? I've had less luck with lightning connectors. But I can plug a drive into my iPad's c port and it shows up in the Files app. I can tap on a picture and then share, and it offers to Save Image, which means to save it in Photos. You can select many pictures to do all at once. But I don't see a way to save Folders of pictures.

If the folder structure looks sufficiently like a memory card from a digital camera, Photos should be able to import the photos directly.

Unfortunately, there is no documentation of what constitutes "sufficiently like a memory card from a digital camera" these days. Originally, it meant one top-level folder named DCIM containing a single level of folders with short names like 101CANON, 102CANON, et cetera containing relatively small clusters of photos (perhaps 100 per folder, more for later cameras) with simple 8.3 file names. As cameras evolved, Photos may have become smarter about what it recognizes, but I don't know of documentation of the bounds of what it will accept.