iOS is Apple's operating system, primarily used for iPhone and some older Apple hardware. Unlike Android, which is used on a number of devices across many different developers, iOS is Apple exclusive. This exclusivity also extends to apps, which can only be bought or downloaded via the App Store.
In 2019, the monthly subscription Apple Arcade began service. It offers a selection of games that can be played across all compatible Apple devices while subscribed. Many of these games are exclusive to the service and can't be bought separately — while some games have eventually made it to other platforms, there's no sign of games like Fantasian doing the same.