You can add playlists that you've created, playlists that other creators have made and videos that you've added to Watch Later to your You tab. Saving playlists allows you to easily find and view them later.
If you own a playlist, you can publicly add it to your library so that your viewers can add it to their own libraries. You can update your privacy settings to remove your playlists from view. By removing them, you can still see saved playlists in your channel but other viewers can't.
YouTube app
Add a playlist to your library
From a channel page
- Tap the PLAYLISTS tab to view playlists for that channel.
- Tap More
next to the playlist details.
- Tap Save to library.
While watching a video
If you're watching a video that's part of a playlist, tap Save 
View playlists that you've added to your library
To view playlists that you've saved and created, go to the Library tab 
Mobile site
Add a playlist to your library
While watching a video
While you're watching a video that's part of a playlist, tap Save 
View playlists that you've added to your library
You can view playlists that you've added to your library and created by tapping the Library tab