I don't consider "Android" to be an OS. It is like Linux, there are a hundred different OSes all running on top of the same kernel and framework. For example, the OS Samsung runs on their phones is so different from the OS Xiaomi runs on theirs that a casual user might think it's a completely different system.

So lumping together all of these variants and calling them "most used OS" is not useful in any sense. They are very different for users, and very different for devs.

This is specially relevant when you want to develop a game against a graphics API. Samsung has Vulkan on their flagships for a couple years now, but not the mid/low-end devices. Pixel supports Vulkan, but the units sold are so low that they are a rounding error. No other major manufacturer significantly supports Vulkan either. In this and similar scenarios, the framework used by ios may well be the "most used" mobile framework.