So given that there is an entire tag on it, that would suggest that people are getting away with serial upvoting. How do they do it?
Simple answer - They don't get away with it.
Votes are always getting annulled and users being removed. If users repeatedly vote for the same user/users then they will get detected and they will get dealt with.
If they're just voting on lots of posts and lots of different users, then it won't be dealt with. But then if that's the case is there a problem there that needs dealing with?