You can't directly browse iTunes/Apple Music app playlists in rekordbox for iOS/Android, but you can use them via rekordbox for Mac/Windows in the following way:
- Right-click the playlist displayed in the [iTunes] category in rekordbox for Mac/Windows and click [Import Playlist].
-> The playlist is copied to the [Playlists] category. (Please note that copied playlists are not synced when you change your iTunes/Apple Music app playlists.)
- Connect rekordbox for Mac/Windows and rekordbox for iOS/Android to the same Wi-Fi network.
- In rekordbox for iOS/Android, tap [Connect to rekordbox (Mac/Win)].
- In rekordbox for Mac/Windows, click [SEARCH MOBILE].
-> The playlist copied in step 1 will be synced to rekordbox for iOS/Android.
- Right-click the track you want to use in rekordbox for Mac/Windows and click [Mobile Library Sync] > [Transfer to the mobile device].
-> The track will be transferred.