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.
That is one of the reason which have got me thinking why certainly it is not easy to start a betting business having relatively small money to start it, specially if the markets who is offering are unknown for the potential bettors who would be willing to give a small bookie a try to begin with...
That is why only big and well established casinos or sport books are afford to offer markets which are rather a niche for bettors, like political events, fashion events, geopolitical happenings a things like those.
Polymarket is actually a good example of a website which have the volume and hence enough liquidity for bettors to open markets which are completely unrelated to sports, which is kind of refreshing if you think about it.
There are people like me who does not have much of capacity when comes to sports, but we keep an eye on geopolitics and the relations countries have omwith one another, those websites give us a chance to capitalize on your talent to analize other things outside sports.