Codes open the file manager on the phone

I want to open the phone’s own file manager in my application and select a file. How do I do it? Below is a sample video of my question

Use the activity starter

I didn’t quite understand how to do it. Can you help by explaining?

read the document and find chapter “Starting a Built-in Android Application from your App Inventor App”

there even is an example " Example: Starting an external app to pick files"
and there is chapter " Discovering how to set the ActivityStarter properties"

I think, it does ot make much sense to cite from that document here, does it?