It is not iOS. You cannot delete it, and you rarely update it manually. It is the first code that runs when you press the power button—before the Apple logo even appears.
For the security researcher, the iPhone 13’s firmware is the Mount Everest of hacking—a silent, black slab of silicon that, so far, refuses to be conquered. Firmware APPLE iPhone 13
When Apple announces a new iPhone, the spotlight almost always falls on the System on a Chip (SoC) —the A15 Bionic, in the case of the iPhone 13 series. We hear about the 6-core CPU, the 4-core GPU, and the 16-core Neural Engine. It is not iOS