I find booting, grud, uefi and related technologies are one of the most complex and hard to make sense of parts of modern systems.
https://en.wikipedia.org/wiki/Das_U-Boot + https://en.wikipedia.org/wiki/Devicetree is an alternative, but it's a massive pain because you lose PnP.
And I'm still wondering about that and how text-mode/console output works.
I get it when I'm attached via a serial port, but... how many console rendering implementations for VGA output are on my system?
Load the first sector of the chosen boot device into memory at 7c00h, and jump to it.
That's how all PCs booted before the monstrosity known as UEFI came into existence.
Yeah, that's the literal handoff, but a LOT of things need to happen before that.
Even before UEFI, you still have to deal with ACPI and friends.
After CPU setup, memory training, dealing with ACPI shit, loading (and executing! [1]) Option ROM, and others?
There are things called "BIOS" doing this before UEFI came into existence.
Why can't we get rid of MMX instructions?
(But yes, booting remains a pain)
Regarding modern PCs: Even I think UEFI is in large parts over-engineered crap it made at least booting simpler and much more sane.