I would like to know how I can create a file selector (for example, return a list of the selected ones) and another dialog box that allows selecting a directory (which returns the direct path to the directory).
I think you are trying to understand B4A vis-a-vis Visual Studio or related thing.
The Form designer can be opened from the "designer" menu, but you will not get a default "form" here with some built in buttons.
You have to "add" the "panel", "buttons" etc. yourself.