Pre-made means that Tidal has made a playlist for you based off of your library, they will introduce new tracks and new artists. Each of the playlists is based off of a general genre (you are not going to find an old country track mixed in with a new heavy metal track). If there are 15 tracks in your playlist, that's what you get, 15 tracks. Yes the tracks in the My Mix will change every once and a while. You can take the different playlists and put them together but Tidal doesn't make it easy to do depending on which device you are using. Amazon's 'My Soundtrack' is a radio station based off of all my tracks (including all genres). When I start playing the 'My Soundtrack' station, it will play a continuous stream of mixed music until I stop it, it'll play mixed content for over 12 hours if I wish it to do so. I get out of bed before 4:30am and that early in the morning I just want to push one button to get my stream of music playing (this is Amazon's My Soundtrack), I do not want to go to several different playlists and have to put them together and mix them myself every few hours (that is Tidal My Mix). But, to each his own.