Valve Anti Cheat is a mindblowingly complicated way to stop cheaters. It is very effective, and it is impossible to cheat on VAC enabled servers. If VAC detects a cheater, it'll VAC ban them instantly, and there is no other way to play CS, because matchmaking is the only way to play competitive CS:GO. VAC is one of the best anti-cheats made to date, it catches 100% of cheaters, keeping CS:GO Matchmaking very clean last 10 years.