The example you can follow to read and write a file on external memory (in addition to the ones you have already seen) is this:
This is a B4i and B4A example, which demonstrates various external files related tasks. The behavior is not exactly the same as the platforms capabilities and behavior are different. B4A Load external files, including online files, using ContentChooser. Save to an external target, using...
Although I believe you've seen it before. I think I understand that you don't want to access a single file but a folder. You can select a single file or a folder, through in ContentChooser.
If I wanted to access a folder containing images in external memory, I would use the system's internal gallery app. Obviously those images should be contained in the gallery.
I don't know if there is a better system, maybe Erel can suggest something better
Finally to have access to the entire memory you have to happen through the file manager as in the photo. Because it has system permissions to access each folder.
If the fileManager does not appear immediately among the options, you will usually find in the menu the possibility to access further folders / further choices
PS. What version of Android does the device you use have installed?