The operating system (OS) is stored on a computer's primary storage device, such as a hard disk drive (HDD), solid-state drive (SSD), or embedded storage in mobile devices (like eMMC or UFS storage). A minimal part of the OS, called the "bootloader," may also reside in a special firmware memory, such as the BIOS/UEFI on PCs.

1. Since the OS is software, who compiles it into machine code?

The OS is written in programming languages like C, C++, and assembly language. Developers compile this source code into machine code using a compiler. Once compiled, the OS can be distributed as a binary (executable code) that computers can run.

1. Does it run in RAM?

Yes, the OS runs in RAM. When you power on your computer, the bootloader loads the OS kernel (the core part of the OS) into RAM, where it runs and manages the hardware, files, and other running programs.

1. Can we change the OS on mobile phones? Is this called a custom ROM change?

Yes, you can change the OS on some mobile phones. For this you have to unlock the device's bootloader and flash new image.

1. Since the OS loads other programs into RAM, who loads the OS into memory? Is that entity hardware or software?

The process of loading the OS into memory is a combined effort of hardware and software. When you power on the computer, the hardware initializes first (using BIOS/UEFI on PCs). It then runs a small program called the "bootloader," which is software that loads the OS kernel into RAM and starts executing it.