The chooser shows a scrollable row of folder buttons and a list of files with icons. Files which appear to be media files, or are too large will be disabled and not selectable. Touch a folder button or folder to change folder, or a file to open a file. Hidden files beginning with a '.' will not appear.
The parent folder, if it exists, will appear first in the list. Touch
that folder to move up the directory tree. If a folder is not
accessible, the chooser will show the parent folder, if it exists, and
the external storage folder (/storage/emulated/0).
Use the Storage button to open files using the android file manager. This should ensure that files on removeable SD cards can be saved using the Save button. The file manager may refuse to open some types of text files not recognised by android.