Sounds like a stupid question though....

I'm a beginner with ableton live 9 and i'm learning new stuff everyday.. I'm trying to produce and perform music with ableton and launchpad. While I was trying to follow what lots of video showed, I was curious how some people can perform the music exactly same as the original one. I mean, how can they perform skrillex music thru user 1 mode? are there samples pack made by original producer or do people produce sample pack just on their own? or are there any music analyzing programs?

If they produced the track themselves, then they will just use those parts. If the track is by someone else they can chop it up a bit or maybe use remixstems. Most of the time they just sample snippets or parts of the song.

Skrillex is not a very good example, when he used Ableton he only mixed tracks with it and maybe drop some samples. If he did something live at all. Nowadays he is all about CDJ's I heard.