From what I know, VAC (Valve Anti Cheat) just looks for processes running on the system and detects injections into CSs memory. Then, for CS:GO specifically there's Overwatch, in which players look at other reported players' gameplay to determine whether they were cheating, and VacNET which is a machine learning system trained on the data from Overwatch to detect aimbots that way. There's a really good talk that someone from valve gave about 3 years ago[0].

The bigger problem is that even with input recognition, one of the biggest problems are wallhacks, meaning you can see other players through walls which is an advantage that's almost as large as aimbotting in tactical shooters like CS.

[0]: https://www.youtube.com/watch?v=kTiP0zKF9bc

The comment above was specifically about aimbots, i.e. cheats that mimic a person. Such cheats can be hidden from the system well enough for it to not know there is a program controlling the input. I'm not saying all kinds of cheats can stay 100% undetected and functional with enough effort on the hackers' side, that's obviously false.