Plenty of good links here already. I'll just add that booting is enormously complex, and starts well before any of the dozen comments so far. I skimmed many of

Plenty of good links here already. I'll just add that booting is enormously complex, and starts well before any of the dozen comments so far. I skimmed many of them (not all) and they all tell pretty much the same story whether on real hardware or virtual. But modern real hardware first boots a Management Engine (or BMC, or T2, or similar), which prepares things and starts subsystems like hardware controllers, before releasing the CPU where the "OS" finally boots, blissfully unaware that it is running on an abstraction. Watch https://www.youtube.com/watch?v=36myc8wQhLo https://www.youtube.com/watch?v=36myc8wQhLo from about 10:00 to about 20:00 for a decent super high level overview. These two are also good for a deeper understanding. https://i.blackhat.com/USA-19/Thursday/us-19-Davidov-Inside-The-Apple-T2.pdf https://i.blackhat.com/USA-19/Thursday/us-19-Davidov-Inside-... https://i.blackhat.com/USA-19/Thursday/us-19-Krstic-Behind-The-Scenes-Of-IOS-And-Mas-Security.pdf https://i.blackhat.com/USA-19/Thursday/us-19-Krstic-Behind-T... They both cover UEFI, even though the focus is on T2. I know that's well beyond what you were asking for, but the boot process goes well beyond GRUB.