Each app or game, which runs on iOS devices, should pay Apple 30% of their revenue. These are billions of dollars each year.

The Fortnite game wanted to avoid this fee of 30% by making the game free to download, but the users could pay directly inside the game (the payment mechanism did not go through Apple).

If more apps or games do that, Apple would lose a lot of money. Also, Apple App store is the only way to get apps and games into iOS devices. So if I make a game and I want you to play it, and you know me and my game and you want to play it too, Apple is a "third person", who can stop us from doing so.