I guess that the way that people have used the clouds won’t work eventually (from Android 11 onwards, unless Google revert the change, which seems unlikely)
From my point of view, they want the apps to have ownership of their own files. If you want to access to ownCloud data, you’ll have to request access to ownCloud.
The basic way is that you open the ownCloud app, access the file you want, and use the “open with” or “send” actions.
For the “open with” action, I guess that ownCloud retains the ownership of the file even though a 3rd party app is accessing to it.
In case of the “send” action, ownCloud gives up the file in favor of that 3rd party so it can save it in the 3rd party’s personal space. Maybe not the file but the contents of the file.
(I’m not an android dev, so I might be wrong, but I think this is the intention)
The other option is that the 3rd party requests access to the ownCloud FS somehow. Assuming that ownCloud is prepared, it’s the 3rd party app the one that needs to initiate the action.
In this case, the 3rd party app would need to know what apps can save the file, or what apps provides a FS. If the app isn’t prepared to ask for that info, then ownCloud can’t do anything.
Basically, as far as I understand, it’s either the ownCloud asks which app can open a particular file (which should be happening already), or the app asks which app can save the file. I think those are the 2 ways the apps are expected to interact with eachother.