Hello,
Please help me by telling how to do it.
I don't think it is possible if you don't know the package name of default/installed File Manager.
I know.
Can u suggest code for that.
... or in App Inventor?
Answer: No.
Something to look at:
How to pick a file
Taifun
Trying to push the limits! Snippets, Tutorials and Extensions from Pura Vida Apps by 
Taifun.
Nope, viewing a file and picking a file are different.
Please explain, what means "viewing a file".
The topic is "How to open a folder ...?"
Viewing a file means you know its path and want to see it either using another app or your own app.
Picking a file means you don't know its path and want to get that either using own app or another app.
In other words, when you use an Image component, you are viewing content located at that particular path.
While when you use Image Picker, you are picking an image from file system/storage and you get its path in return.
Yes, I know that. But what does that have to do with the fact that there is no way to open a specific folder in AI2, Kodular or any of its other clones?
Of course, you can list files from a specific folder:
You can open specific folder in a file explorer app if you know some details about that app such as package name.