Both Netflix and Youtube (premium) can be used completely offline. I would guess that the iOS and Android apps have a pretty encompassing local data model to ke

Both Netflix and Youtube (premium) can be used completely offline. I would guess that the iOS and Android apps have a pretty encompassing local data model to keep track of everything from watched status to settings changed in the preferences. Not to say that it isn't synced with the server, but the server isn't the only representation.