You turn on a computer. What happens first?

The operating system loading is a much later stage, occurring after the BIOS has identified a bootable device and executed a bootloader.

The boot sector is accessed by the BIOS or a subsequent bootloader after the BIOS has initialized and identified bootable devices.

The BIOS (Basic Input/Output System) is firmware stored on a chip on the motherboard. When the computer is powered on, the CPU's initial instruction pointer is directed to the start of the BIOS code, which performs the Power-On Self-Test (POST) and initializes hardware before any storage devices are accessed.

The Master Boot Record (MBR) is a specific type of boot sector on a hard drive that contains the bootloader code; it is accessed after the BIOS has performed its initial checks and identified the boot device.