Just paying or instructing the ref alone can already change the outcome of a game.
This is what some people might miss, they think that fixing a game means everyone has to be in on it, which in reality isn’t possible because it would be too easy to find the mastermind and put them in jail. I see it as a serious case since it’s basically a betrayal of public trust.
Getting the refs involved is the easiest way to rig a game. At the end of the day, they’re the ones making the calls on what they see, and they can be biased to give one team an advantage and secure the win.
As for major leagues, I believe some fixing might be happening, but that’s just my speculation. It will never be proven because if it ever was, the league would be forced to shut down, and the business loss would be massive, other major leagues will be affected.
It is easy - just get insider information.

It seems to me that everything remains the same - rely on your luck.
But its not easy to get this information.