I am not sure if I understand correctly but if I somewhat understand it correctly. There are usually 2 ways in dealing with this.
1) Server side where a user has an account, is logged in and retrieves that data from the database.
2) Use cookies to mark something as followed, watched, add to playlist, likes etc.. in which all these would work with option one as well.
I have seen more and more places use cookies since users want interaction but not to always sign up.