How to develop file manager/explorer for android devices?
If you are talking about an app like Total Commander that should also have access to the ASD of all other apps, then this is definitely a question for very advanced app developers.
In addition, in order to have access to all external storage directories, a special permission (
MANAGE_EXTERNAL_STORAGE) must be requested, which is usually not allowed by Goggle, unless one explains and justifies why the app needs this access as a core function of the app.
You still hace access to media files... you might want to try the Jukebox example here App Inventor Extensions: File | Pura Vida Apps
Sorry, but it does not work like that...
It looks like you have do try something yourself. .
A very good way to learn App Inventor is to read the free Inventor's Manual here in the AI2 free online eBook App Inventor 2 Book: Create Your Own Android Apps ... the links are at the bottom of the Web page. The book 'teaches' users how to program with AI2 blocks.
There is a free programming course here Course In A Box and the aia files for the projects in the book are here: App Inventor 2 Book: Create Your Own Android Apps
How to do a lot of basic things with App Inventor are described here: How do you...? .