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:
Tag: Howard Oakley
Apple macOS automatic EFI checks
https://twitter.com/osxreverser/status/912014988608491520
High Sierra automatically checks EFI firmware each week
Upgrading to High Sierra brings a new and significant security feature: your Mac will automatically check its EFI firmware. In a series of tweets, Xeno Kovah, one of the three engineers responsible for the new tool, has outlined how this works.[…]
AFAICT, the article references Tweets from earlier today that appear to have subsequently been deleted from Twitter.
Firmware Security 