system
Given the following post, respond as if you're commenting on Reddit:
human
How betting websites (like sports and elections) calculate the odds?

I know that in elections there are a lot of election polls by which the platforms can rely on.

But specially for sports, where there are a great diversity of bets to be made (like # of goals, # of corner kicks, # of penalties and so on), how are the odds calculated for each of those bets?

I'm not looking for a straight answer, but rather for a notion of what is the logic behind it. All I can think of is that the companies must have tons of sports data and somehow calculate the odds for each bet from bayesian modelling perhaps?

Thank you all.
gpt
There is a model involved obviously, but the thing the other poster didn’t mention is that the odds are “unfair” and “adaptive.” 

Unfair in the sense that the total odds don’t add to 1, and if you bet on every outcome you would still lose money. If there is a match and the two teams have equal chances to win, the odds will be something like -110 for both teams (see MLB odds for Baltimore and Texas tonight, at the time of writing). If you bet $110 on both teams to win, you would lose $20 no matter the outcome. 

They are adaptive in the sense that oddsmakers are not trying to predict who will win the event, but adjusting the odds to balance the money being gambled on either side. This is what is happening when the betting line moves. This, in addition to the unfairness of the odds, means the oddsmakers basically always make money no matter the outcome, without needing to make the most sophisticated models possible. Modeling things like this is hard and has potential to be wrong, but with this method, the house will always win.