While it's nice to imagine it to be this way, games never come fully-formed from the mind of a designer/developer. A game is made up of many different ideas from many different people. While in the past people could develop games with only a single person, now it's much more common for teams to be made up of many different disciplines and every game developer on your team has ideas, many of them good that can contribute to the final product that gets made. But with that in mind you may be wondering how does a game get to the final point with all those different changes made? The answer to that is iteration.