When you bet on Sportsbook you are betting against Betfair rather than against other Betfair customers. Therefore it is Betfair that set the prices and carry the risk.
To calculate your potential payout on an underdog, all you need to do is multiply your stakes (the amount of money you wagered) by the value resulting from the moneyline odds divided by 100. Put simply: Potential profit = Wager x (Odds/100).