The first-stage bootloader, eg BIOS, looks for a second-stage bootloader to load on a disk.
The second-stage bootloader loads the linux kernel then runs "init".
GRUB is a second-stage bootloader.
If the drive is partitioned using MBR, it is stored in the MBR.
With BIOS and GPT, there needs to be a separate boot partition for it. With UEFI and GPT, it can sit in the EFI partition.
There are config files associated with GRUB:
/etc/default/grub
/etc/grub.d/
Running update-grub can reflect changes in the boot path.
Allows EFI firmware to load kernel as EFI executable.
Run memtest from grub