A: Apple is limiting file access to the apps therefore apps can not access files store somewhere else on your Apple device.

There are 2 possible approaches:

  1. iTunes library: create source entry pointing to your iTunes library and sync your music files via Apple iTunes PC app. The main limitation of this method - you can not sync files which are not supported by iOS, for example DSD files, APE, WV, and so on.
  2. App's /Documents folder: every app on Apple device has its own Documents folder. It is app's internal folder where you can drop files which can be easily accessed by the app. This approach supports any type of the audio file, so it is suitable for DSD, APE, WV and others.

For the App's /Documents folder case you need to create source entry pointing to the Internal Storage (Documents): go to media library screen -> scroll to its bottom to Sources -> go inside Sources -> press [+] button in the bottom panel -> File System -> select Internal Storage (Documents) -> confirm.

Then you can drop a whole music folder via Apple iTunes file sharing feature. Or the best would be to bind internal FTP server to this source entry and access internal Neutron's file system with any FTP file manager, you can get further details in Q: How to transfer files to Neutron over the local network?.