Linked from
The 8 pages that link to Bootloader, each with the reason it gives.
FirmwareBroader topic: It is often the firmware stage that hands control to the operating system.
Assembly languageRelated: Early startup code may run before ordinary compiled software is available.
BIOSRelated: BIOS transfers control to boot code that begins loading the operating system.
UEFIRelated: UEFI launches an operating-system bootloader rather than loading the kernel directly.
System softwareBroader topic: It bridges firmware startup and the operating system’s launch.