This is an Android issue rather than a Kivy issue.
I wondered about this too, as far as I can tell:
App shared storage was depreciated in Android 9 , and removed in Android 10. The two app private locations are unchanged.
So presumably global shared storage must be used.
Shared storage such as Downloads is accessed via an Android intent (see Android Developer docs)
It seems what you want to do would be a good thing to figure out one day ;)