Since it's a mac app, it could store the metadata in either a file, like you mentioned, or in a userdefaults preference dictionary (plist) that would be stored

Since it's a mac app, it could store the metadata in either a file, like you mentioned, or in a userdefaults preference dictionary (plist) that would be stored in the app's library support folder, or they could use core data and just keep a unique identifier for the path to the folder