Basically, I am having huge difficulty getting a text file to open by my UWP app. I have set the app permissions to be able to access files in \'my documents\' and other opt
Yep like you've already known, a common UWP app cannot directly access some paths that is not allowed. You need a broker to help you access it using FilePicker. I believe you can found lots of articles about this.
So why you can see some notepads in the store which seems different? This is because those apps are converted apps which used DesktopBridge technology. Desktopbridge allows common desktop apps to works in UWP app container and have more privilege. Although it has some problems for path that you need to notice, like the following blogs mentioned: blog1 and blog2.