Modern casino slots are controlled by computer chips and software, and also the win percentages are preprogrammed in to the software.

Casino slots have Random Number Generators (RNG) that are continually generating combinations, even when the machine is not being played.

Running concurrently with the RNG of the casino slot machine will be the payout percentage.

The payout percentage controls the amount the slot machine will payout, for example the payout percentage might be 90%, meaning the slot game will payout 90% of all of the bets played and also the casino only gets to keep 10%.

This is not to say that every time you bet 10 credits you shall get 9 in return - where will be the fun and excitement in that? Instead the casino slot machine game will not pay out for a while, source website and then suddenly go on a hot streak and you might hit a big jackpot.