See Boot Methods.
Boot loaders start after bios/uefi post. They load the kernel & start the system, and hand off from kernel to init. The traditional personal computer architecture loads only the first 512 bytes from a disk into memory and begins execution. Operating systems are larger than 512 bytes, so a bootloader deals with loading its own components as well as the operating system into memory. The original bootloader contained enough code to determine the active partition of an MBR disk, load that partition's volume boot record, and pass execution to it. Modern bootloaders are much more capable almost forming operating systems of their own merit. Funtoo offers Grub as its preferred bootloader though there are several alternatives available.
uefi & bios