When you install a game on your iPhone, or indeed any app for that matter, you do it from the App Store. That store is controlled by Apple and everything you download from it is vetted by Apple ahead of time. But that also means that Apple gets a cut of everything you buy there. Game companies like Microsoft's Xbox and Epic Games don't like that idea for obvious reasons.
It also means that anything that Apple doesn't want in its store, isn't. And if it isn't in the App Store it isn't on the iPhone unless it can be run via a web browser instead. That's famously how Microsoft was able to bring its cloud gaming service to the iPhone after Apple refused to allow it into the App Store. And it's Microsoft, via its Xbox division, that is getting ready to change how things work for good. If it's allowed to, it intends to offer an app store of its own.
