pressing play on a mac is mixing?
lol you have no idea do you
my friend owns a traktor s4, a simplified version of what skrillex and other producers use.
skrillex uses:

^ or something similar, some are even more complicated. this is the mixing table alone.

^ this is what he uses to launch specific samples. he plays as he goes.
not to mention that he's already MADE the music. in live sets he modifies and mixes the song as he goes.