Up until the last season, the NCAA did not allow helmet communication, so they had to use signs that had symbols which indicated what play the coaches wanted the players to run. ‘Decoding’ these during the game is not illegal, but a u of m coach was caught sneaking around and recording signs and their associated plays which is very much not allowed. This allowed them to run the exact right play to counter whatever the other team was doing, which ultimately led to them being able to win an (undisputed) national championship for the first time post integration. There are many other wild and crazy things that are a part of this scandal, if you want to learn more, simply look up “Michigan sign stealing.”
I would say the Astros were worse because they were only caught after they had won the World Series, while u of m was caught mid season before they went to the championship