it definitely is complex. TLDR TIME... you've got high, mid and low attacks, throws, throw breaks, reversals, counter reversals, parrys, launchers, air combos, combos after you've crushed them into the ground, combos after you've smashed them into a wall, okizeme (the art of kicking them while they're down and not letting them get up), you've gotta recognize every character's entire move list and react accordingly, duck highs and use while-standing-punishers, block or sidestep (in the right direction) mids, block lows or better yet use a jumping mid attack, know frame advantages/disadvantages of certain moves (when an attack is blocked or whiffed, it has a time window of a varying number of frames depending on the attack, where it can be punished with a move with a less than or equal amount of frames) etc etc.