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.
But now I think most Casinos are doing both, so if you are betting in sports they will find a bettor that nearly place the same bet as you and they will either fill your over/under bet or they will do it on the other side. Basically, we are just against other players and casinos are just filling in some gaps, but if they cannot find anyone, or you are just the only bettor they will just play against you.
I think this is how it goes.
If Drake bets 100K - Casino will find 5 bets with 20K each. If they cannot, then they have to play against Drake.