Apple is often credited with creating the integrated app store for the iPhone, but it's not really an accurate remembrance of history. In reality, the iPhone launched without the ability to use 3rd party apps. All that was available were Apple's small number of built-in apps, and 3rd party developers were encouraged to build mobile web apps that would run in Safari. However, users looking to upgrade their iPhone from a media device (the initial categorization for the iPhone) to a smartphone quickly learned how to jailbreak their devices. This allowed for app stores, such as Cydia, to be installed onto the iPhone, bringing app installations to the platform.
Cydia was available long before the official App Store and made it possible to install additional apps onto the iPhone and iPod Touch. Once the App Store launched, Cydia continued to evolve, adding the ability for users to customize their lock screens and home screens in ways that Apple did not allow but the operating system did. Over the years, Apple has implemented some of the features from Cydia, like a quick launch for the camera, all while preventing users from using officially using Cydia.
Following the trend in the industry, Cydia has sued Apple, claiming that the practice of preventing the alternate app store from being installed on the company's devices, despite being fully functional and popular, is illegal anticompetitive behavior. The lawsuit states,
Were it not for Apple's anticompetitive acquisition and maintenance of an illegal monopoly over iOS app distribution, users today would actually be able to choose how and where to locate and obtain iOS apps, and developers would be able to use the iOS app distributor of their choice.
Cydia is not the first to make this claim. Epic Games has made this same argument, stating that Apple is free to prevent apps from entering its own App Store, but only if they make alternates available. Otherwise, blocking any apps, like Fortnite from the App Store can be seen as anticompetitive.