Sync/Import File

Hi, I was wondering if there's any way to sync library across devices.
Ideally I think a good way would simply be able to import/export files from/to external folders.
The custom folder function currently does not seem to read any existing files in the pointed folder.
I've tried extracting the files from the app's data folder and copy to the same folder on another device but the app does not recognize those files (not shown when open library tab)

Thanks