Idiskk 1 TB external hard drive

I purchased a 1 TB idiskk to backup photos. It worked very easily and quite fast. I backed them up from my new iPhone SE. The app works well. I also have an iPad Air 3 I’ve had for a relatively short time. And an iPad mini.

What I can’t figure out is how to backup my graphics files essentially and miscellaneous in the Files App. I am able to do so one at a time through sharing but there just HAS to be a far less labour intensive way of doing so.

I do have a dongle I use with flash drives.