Chooser

In your subject, you seem to be referring to the "Dropbox Chooser", which is a pre-built way for developers to easily allow their end users to choose files or folders from their Dropbox accounts to give to the third party app. If you're interested in using that, you can find more information here.

From your description though, it sounds like that may not be quite the functionality you're looking for. If you want to be able to programmatically list the contents of a folder, such as the root folder, you'd need to use the Dropbox API instead. The Dropbox API lets third party apps list, upload, and download files and folders, among other operations. You can find everything you need to get started with the Dropbox API, including documentation and tutorials, here. For instance, to list the contents of a folder, including the root folder, you'd use the /2/files/list_folder[/continue] endpoints (or corresponding methods in an SDK/library). I also recommend reading the File Access Guide as it sounds relevant to your use case.