Fixed Odds Horse Racing

I am familiar with horse racing as it is booked in the U.S., but I do not understand how fixed odds horse racing works in other countries.

Can someone explain to me how the horse track makes money by offering fixed odds and what is the typical hold percentage?

Also, what do you think the hold percentage of the Sigma Derby games is set at?

Do any tracks themselves offer fixed odds? Usually, there's an area where bookmakers set their own odds (and change them on the fly to balance things out). Do a search on "tic-tac".