@AB1
So, generally, you will first connect to your Library in your data sources. If it does not appear in the list of "lists" in your SharePoint site, then you can type it in directly into the empty box in that list.
To get you started - for your Gallery, you can use a formula like this:
Filter(yourDocLibrary, IsFolder)
Put a label in the Gallery and set the text property to ThisItem.Name and you should see all of the folders in your Library.
This will show all of your folders in the library (note: all of them...not in a hierarchy)
There's a lot more to do after this to get to an "explorer-like" experience. There are some posts on this forum where people have done it fairly well.
A bit of advice though from an App perspective...if you can get away from the traditional explorer-like, hunt and find process for users, the better received your app can be. With Document Libraries, you can establish Metadata on all of your files and capitalize on that for your app experience. In example, if you are dealing with a Library of contracts, you can have customer information in the metadata and document types and status as well. This way, a user of the app can choose a customer and then see a list of all of the documents associated with that customer and even filter down to the type of document and the status of the document...the possibilities are endless with this approach. The other nice thing about this is that you typically end up filtering on columns in your library that are delegable - and thus no issues with delegation limits. Just an extra $0.02 to keep in mind.
