Re: Dual booting Debian on an Windows machine.

[IL Ka <kazakevichilya@gmail.com>]

  https://uefi.org/specifications
current is UEFI Specification Version 2.9 (March 2021)
  https://uefi.org/sites/default/files/resources/UEFI_Spec_2_9_2021_03_18.pdf
although i still read
  https://uefi.org/sites/default/files/resources/UEFI_Spec_2_8_A_Feb14.pdf
when it comes to discussions.

You are right, but OP asked for "newbie instructions" on how to multiboot Debian and Windows.

 

> Harddrive is GPT-labeled and it has a partition with a special EFI flag.

It may also be partitioned by an MBR partition table.
(UEFI 2.8, 5.2.2 OS Types:
 "0xEF (i.e., UEFI System Partition) defines a UEFI system partition."
)

AFAIK it is up to firmware to support MBR or not. Even more fun: firmware _may_ support any filesystem including NTFS or ext3, but every modern firmware _must_ support GPT and Fat32.