Where does Kali declare and set $prefix for grub2 ?

In my install of Kali, the grub2 variable $prefix is set to (hd0,gpt3)/@/boot/grub

I need to alter its value for a system configuration.

Where is it declared and set to (hd0,gpt3)/@/boot/grub ?

Places I have looked:

• /boot/efi/EFI/kali/ (there is no `grub.cfg` here)
• /etc/grub.d/
• /boot/grub/
• /usr/share/bash-completions/completions/

A little more system information:

• Using btrfs file system
• dual booting ( Ubuntu + Kali ) from within a single btrfs partition