How to read/write external files?

Platform: Android

Hello, I would like to know how/if it is possible to read/write external files that are not part of the app. I want to access folders such as downloads, images folder, documents folder etc. I need to be able to read/write/delete the files in these folders. I checked the FileSystem docs but they say that it is only possible to access app folders and not any external folder.

Thanks!