Thank you Steve, I will follow these links. My question was indeed too large. Here is what I am looking for :
I would like to build an APP where the user can:
Activity 1 : Search and pull out of his or her phone (i.e. sdCard) a picture or a file and store it in the APP.
Activity 2 : To WRITE this picture or file to another phone or to an NFC tag.
Activity 2 is easy because it is covered in the tutorials examples but how about activity 1?
Putting the question in a different way : How to make an APP that allow the user to access his Phone's SDcard? Tks.