I have an app that allows me to export (and import) its contents.
While working to replace the deprecated ‘unarchiveTopLevelObjectWithData’ unarchiveTopLevelObjectWithData (still ongoing) I ran into some issues testing.
My test was that I had previously exported to the Files app and from there using the share sheet I sent it to my app. This was not working.
In the past I had imported files from an app like WhatsApp and that worked fine.

After some troubleshooting I discovered that for some reason when sharing from the Files app (and not just iCloud Drive) I get a different URL for the file than I get from other apps.
If my file in the Files app is in iCloud Drive I get the following path:

/private/var/mobile/Library/Mobile Documents/com~apple~CloudDocs

When I move the file to the ‘On My iPhone’ location and share from there I get:

/private/var/mobile/Containers/Shared/AppGroup/***/File Provider Storage/Files

What my app is built to work with is that when a non-Files app shares a file it’s put in an ‘Inbox’ subfolder of the Documents folder. So what my app currently does is check in that location.
Now I’ve modified the app to take the URL from the ‘application:openURL:options:’ method. But when I use that URL to check the existence of the file using ‘FileManager.default.fileExists’ it can’t find it.

So my first question is why the Files app works differently from other apps when it comes to how it shares its files?

The second question is what do I need to do to allow my app to access the urls produced by the Files app?