Removing an instrument from a track is accomplished in a couple ways.
The most popular method is to reverse phase the stereo so that anything dead center (typically the vocal) within a certain band is canceled out leaving only a trace of the original. Anything panned to the right/left or outside the defined frequency band remains. This method is wildly popular for removing vocals and guitar leads.
Drums are across all frequency bands, and typically are not all mixed dead center, so this trick doesn't work for them. You might have some success removing one instrument at a time (kick/snare), but a lot will remain.
The drumless tracks that you find on youtube are typically Rockband-rips and licensed-for play-along tracks.
Those instrument isolations people post are from the Rock Band video games. You can extract the individual files somehow. The only songs people can do this with are the ones on the games. It isn't a secret trick.