Reposting some old comments on UEFI, since I got riled up about it again:
UEFI is perhaps the single most egregious pile of garbage ever concieved by humans. I don't think it's possible to overstate how awful it truly is.
Here's how BIOS works: basically, the computer loads the first 512 bytes of disk into memory and jumps to it. The OS takes over from there and does whatever it needs to, like bootstrapping stage 2 and loading the kernel from the filesystem. The spec[0] is 46 pages long and you are granted the rights to reproduce, distribute, and implement it free of charge.
Want to know how UEFI works? The spec[1] is a 2,899 page PDF which you have to pay to use in any way, including writing an implementation.
Every copy of this specification should be found and burned. The ashes should be buried deep in a dark place and the earth above salted.
[0] https://web.archive.org/web/20110715081320/http://www.phoenix.com/resources/specs-bbs101.pdf
[1] https://uefi.org/specifications