it looks like the question is “how to pick a file”?
there are extensions around
File Picker Plus Extension by Andres Daniel (12 USD)
File Picker Extension by Deep Host
for the extensions directory see here App Inventor Extensions | Pura Vida Apps
and you also can use the activity starter together with a file manager app of your choice, for example like this App Inventor Tutorials and Examples: Pick a file | Pura Vida Apps
Taifun