I don't know how bookmakers will know that the match was negotiated. Perhaps such information appears to the police or somewhere else. Maybe someone made a very big bet on the incredible outcome of the match. There are a lot of players in the contract matches, someone can talk to his wife, she will tell her friend and so the information went further.
Bookmakers receive all information about fixed matches by statistics of accepted bets. As soon as they notice anomalous activity - a large number of bets on a certain outcome of the match, then the match immediately falls under suspicion, if this imbalance continues to increase, they cancel the match as suspicious. Statistics let you see everything. And you must understand that all bookmakers interact with each other on this issue. Therefore, using different bookmakers to cheat will not help.
Obviously, you can dishonestly win a small amount without anyone noticing it, but this is not profitable for scammers and athletes who are willing to cheat.