I know that if you are just working within a small bookmaker or a local one in your place, you are against the Bookmaker, but if it's quite a big audience, you are now against other bettors. The role of the bookmaker there is to adjust the odds and ensure a balance between both sides of a sure bet.

It also depends on the market that you are betting with. I think in polymarket, it's within other players. Based on what I have read, I haven't tried it, but it's a fantastic site.