How do you cheat in world class level chess?

I used to play chess in a completely amateurish way decades ago, but I really don’t understand the game beyond the basics, so this story puzzles and fascinates me:

The cheating controversy gripping the world of elite chess was already enigmatic — but it deepened even more this week, when world champion Magnus Carlsen abruptly resigned after making a single move in his highly anticipated rematch with Hans Niemann.

Carlsen, 31, and Niemann, 19, were facing off in the Julius Baer Generation Cup roughly two weeks after Niemann defeated Carlsen — a win that was immediately thrown into question by a cryptic tweet from Carlsen that seemed to suggest Niemann was cheating.

The drama threw chess into a tizzy, and fueled anticipation for Monday’s match between Carlsen and Niemann in the online tournament. But after Niemann made his first move as white, Carlsen responded with a single move as black and then quit.

If I understand the situation correctly, Carlsen is considered one of the greatest players of all time, maybe the greatest. (Before his controversial loss to Niemann he hadn’t lost a game outright while playing white in years.)

What I don’t understand is how one would even manage to cheat in tournament-level chess, since chess isn’t even a game in the game theoretical sense of the word. In game theory a game requires players to have at least some information that isn’t shared with other players. Thus poker is a game in this sense, but chess isn’t, since everything is out in the open.

So how do you cheat?

I don’t know that we’ve ever had a chess discussion on LGM so have at it.