Every game in the casino uses the RNG. It generates random results. Everything in the casino is random.
So let’s say you are playing a 5 reel slot. And let’s say there are 50 symbols on each reel. In order to calculate the result of the spin, the casino software will call the RNG 5 times, once per reel. Each time it will get back a number between 1 and 50. That number indicates at what point each reel has stopped, and therefore it knows what each reel is displaying and therefore it knows what payline(s) have hit.
As I explained earlier, the overall payout percentage for each game in the casino is calculated according to the payout of each individual event multiplied by the probability of getting that event.
A slot machine has many different ‘events’ – for example on Avalon you can get paid for getting 2,3,4 or 5 treasure chests, or 3,4,5 Js, 3,4,5 As, etc. Each of those is a possible event, each has a predefined payout amount, and each has a probability of occuring. The overall % payout is the combination of all those.
So when a casino designs a slot they can’t just say “I want it to pay out 95%”, they have to decide on all the different combinations that will win, and how much each of those will pay when it does hit, and then calculate how likely it is to get any of them, then add all that together and work out how much it will pay overall.
Just to emphasise one last time – casino software does not pre-decide the results of a game. It simply randomly generates a new game result, and then checks its paytable to see how much the player should win (or lose) for that result. Because each result is randomly generated, the laws of probability dictate that over the long run each event will occur the correct number of times and thus the game will payout the amount it is expected to.