When most people think of custom ROMs they think of swapping an official Android firmware build with a custom one. And there’s a good reason for that: Generally, no developers are creating custom ROMs for iOS.
For years people have been having conversations about developing an iOS copy of a ROM on an Android build, but unfortunately, it’s never happened. According to the XDA-Developers forum, it would be too difficult and buggy to pull off.
Here’s the catch though: There is such a thing as custom firmware on iPhone. On Android, we use the two terms “custom ROM” and “custom firmware” interchangeably. I don’t know why that’s different on iPhone but there is no information regarding custom ROMs on iPhone yet there is plenty of information regarding custom firmware. The iPhone 4S came with iOS 5.1.1 out of the box which means you will want to look for custom firmware that is based on iOS 5.1.1 or later. And with a Google search, I can see people developing custom firmware based on iOS 5.1.1 using redSn0w and other tools that are known as jailbreak tools. I can also see iOS users downgrading with custom firmware using Ultrasn0w.