Basic concepts and proper useage of UEFI

[Richard Owlett <rowlett@cloud85.net>]

This post was prompted by recent discussion of problems/cautions 
concerning UEFI. All my machines boot in Legacy BIOS mode and I have 
minimal background in UEFI.

A preliminary web search yielded:
> http://x86asm.net/articles/introduction-to-uefi/

> https://blog.thomasmarcussen.com/tag/configmgr/

> https://blog.thomasmarcussen.com/unified-extensible-firmware-interface-uefi/

> https://wiki.restarters.net/UEFI_and_GPT

> http://x86asm.net/articles/others/index.html

> https://www.thomas-krenn.com/en/wiki/GUID_Partition_Table

I have not yet read these in detail. Is there other introductory 
material that would be recommended.

Also, in https://lists.debian.org/debian-user/2019/05/msg00107.html , it 
was suggested that a separate thread be begun on the question of
  "Whether or not you can update/install UEFI if you are not booted
   into UEFI."

I would like to see that discussed.
I hope I've used an acceptably broad subject line:

TIA