[fratajcz]

Most of them are ripped from Guitar Hero/Rock Band, because the games needed to mute individual instruments when you were not playing the correct notes. (With 10+ games and lots of DLC, there's actually a lot of songs that were included)

There was also a service called jammit marketed to musicians where you could buy multitracks from their partnered artists. There are also artists sharing them in deluxe editions of albums, for example I think Dream Theater did it for Black Clouds and Silver Linings?

There's also the possibility of just knowing someone in the industry that could have access to them, maybe? I know Rick Beato who does song analyses on YouTube is a producer, so he could get some this way.

There was a great channel on YouTube dedicated to sharing these isolated tracks called digital split, unfortunately it got banned from YouTube twice, so I don't think it's coming back...

[bluGill]

It wouldn't surprise me if popular channels got access to the raw tracks from the studio (for whatever the normal license terms are to use the full copyrighted song in your video - a share of the advertising). Popular is key - you need to get popular doing analysis of work nobody has heard of first. Once you are popular on YouTube everyone wants you to advertise for them.