It is typically burned into a tiny ROM chip on the phone's circuit board.
Basically operating system in cell is stored in ROM.
The Android mobile operating system is Google's open and free software stack that includes an operating system, middleware and also key applications for use on mobile devices. There are several types of operating system exist on the internet for mobile devices. The three most popular operating systems are Android, iOS and Windows. What are the different types of smartphone operating systems? A mobile operating system also called a mobile OS.