File storage on iOS is a confusing topic. iOS (and iPadOS which is just a variant) do some clever things in order to limit the inefficiencies of the sandboxing used by the OS. (Sandboxing is a technique that prevents apps from altering data used by other apps.)
In many cases, when you copy a file from one app's space to another, the OS does not copy the data but simply creates something like an alias to the original data. If you alter the file in either app, the OS will then save the altered data into its own file.
This creates a lot of confusion and worry because most people don't know about this and because there are situations where it isn't clear what will happen.
iOS makes a distinction between folders that show up in File in the “On My iPad” or “On My iPhone” directories and those that show up as being in their own file provider such as Dropbox, Documents by Readdle or Audioshare. These are technically File Providers which seem to the user to behave more like a separate drive than as a folder on your device.
In many cases when you think you are copying data, you aren't really. See What Really Happens When You Duplicate a File on iOS for more detailed understanding of this issue.
WIKI USERS: please update this article with more details!!!!
See also AB Forum thread Managing Sample Folders Bewtween Apps Is A Complete Mess.