How do people get their 335i to beastly HP ??

I'm just curious how do people get their cars to those 700+ HP and crazy torque ??

Like those 700HP 335i and even higher , Subaru STI with even higher numbers

How ?? ECU Tune, downpipes, exhaust or intake just doesn't get it there ? So how ?

How do people get their 335i to beastly HP ??