they investigate involving the use of authorities and IP address tracking to find as many people that had the papers. they then disqualify those and can imprison/fine people who shared the papers.
last year they disqualified everyone on some massive snapchat groupchat where the paper was sent. its probably a good idea to report people you know have the paper as it will make it fairer for us.
some people will get away with it, which would raise the grade boundaries. we don't have to retake but for example, last year A* chem was 82% as opposed to like 72% usually, due to the leak.