Here it has been said
SAF can give you access to 90% of the file system.
But you can’t access /Android/data and /Android/obb dir and its childs.
See also SAF: App Inventor implementation of Storage Access Framework - #116 by bodymindpower
However you might want to try SAF: App Inventor implementation of Storage Access Framework - #163 by vknow360, see also the following posts there
Taifun