I think that there is a team full of mathematicians and data analysts that work on making odds and then these bookmakers get odds from these odd providers. It's similar to slots, there is a group like NetEnt that works on slots and casinos hire their products.
Probably, I suppose that nowadays they will be based on the same kind of statistics for the bookmakers, and what we were talking about the balance especially when there are many more bets on one side or someone bets strong, but seen that there is a trend across all bookmakers, like the statistics.