He cannot be overruled. If he says he saw the entire incident and deems it a Yellow Card, nothing can be done. It can only be dealt with by the CO if the Ref says that he missed all or part of the incident.
Games cannot be re-refereed after the event, so it was the case that if the Ref dealt with the incident, by means of a Yellow Card, the CO couldn't take any action and it ended there.
Then, Clerr the Tim got the CO role and found a loophole (almost always used to punish Rangers), whereby if the Officials claimed that they didn't see the entire incident, then the CO could take action and upgrade a Yellow to a Red.
All of a sudden, we had Refs staring directly at incidents involving Rangers players, saying that they never saw the entire incident or if they had seen it from a different angle, they would have made a different decision. Which is why what is or isn't shown on Sky or BBC is very important.
As far as I'm aware, just like Covid suspensions, we are the only country who use that loophole and other countries only acted on incidents completely missed by the Officials (pre-VAR obviously).