When you open a game you like and its rules, you'll find a place that shows you the return to player percentage. For example, the "NetEnt" game "Starburst" has a payout percentage of 96.1 which means that 96.1% of the total amount wagered on the game will be returned to the player. However, this percentage does not mean that each player individually will receive this amount of money back.
The payback percentage applies to the game in general, across the entire network, i.e. to all the people playing the game at different online casinos around the world at the same time. As these games are based on a Random Number Generator (RNG), which is beyond our control, to ensure fair play, the payout of winnings is always random.