I’d be surprised if many sets are premixed, but many are preplanned (as in, the DJ knows which songs he/she will play, in what order, and how many bars out/in to start the fade).

Of course, that’s still pretty boring to watch, which is why there’s a lot of fake knob twists and styling going on.

I consume a lot of Youtube DJ sets but it’s all just playing in the background, I never watch the video.