Boot process

BIOS-MBR boot (Windows on MBR)
BIOS-GPT boot (Windows on Hybrid MBR/GPT – Dangerous!)
UEFI-MBR boot (Windows on MBR, CSM mode)

1. BIOS does POST.
2. BIOS reads MBR (which contains Windows Boot Loader) then execute it.
3. Windows Boot Loader searches active (boot) partition.
4. Windows Boot Loader chain-loads VBR (which contains Windows Boot Manager) of active partition.
5. Windows Boot Manager loads BCD then show boot menu.
6. Windows Boot Manager executes WINLOAD.exe to boot Windows.

BIOS-MBR boot (GRUB on MBR)
BIOS-GPT boot (GRUB on Hybrid MBR/GPT – Dangerous!)
UEFI-MBR boot (GRUB on MBR, CSM mode)

1. BIOS does POST.
2. BIOS reads MBR (which contains GRUB boot.img) then execute it.
3. GRUB (stage 1) reads LBA1 (on MBR) or BIOS Boot Partition (on GPT) (which contain GRUB core.img)
4. GRUB (stage 1.5) reads partition table, and loads /boot/grub/
5. GRUB (stage 2) show boot menu entries.
6. GRUB loads linux kernel image to boot Linux.
7. GRUB chain-loads VBR of other partition to boot other OS.

UEFI-GPT boot

1. UEFI does POST.
2. UEFI reads EFI Boot Partition. (which contains boot loader images)
3. UEFI executes boot loader.