Starting from iOS 11, by setting
LSSupportsOpeningDocumentsInPlace to YES and
UIFileSharingEnabled to YES in the Plist, you can access the app's Documents directory from the system's Files app.
Open from the system's "Files" and view "My iPhone" as shown below. Apps that have enabled this feature can be seen here.
Note: This directory is the app's "Documents" directory, so it will disappear when the app is deleted.