Howard Oakley has yet another new blog post on how Apple EFI works:
Booting the Mac: Will my Mac boot from this disk? A visual guide
There have been multiple recent blog posts on Apple EFI from this author! Eg:
https://firmwaresecurity.com/2018/08/30/booting-the-mac-bless-and-what-makes-a-volume-bootable/
https://firmwaresecurity.com/2018/08/26/booting-the-mac-visual-summary/
https://firmwaresecurity.com/2018/08/26/whats-stored-in-mac-nvram/
https://firmwaresecurity.com/2018/08/24/booting-the-mac-the-kernel-and-extensions/
https://firmwaresecurity.com/2018/08/10/booting-the-mac-loading-boot-efi-and-secure-boot/