Custom menu entries for grub2 for btrfs subvolumes. This file lives in /etc/grub.d/.

40_custom

#!/bin/sh
exec tail -n +3 $0
# This file provides an easy way to add custom menu entries.  Simply type the
# menu entries you want to add after this comment.  Be careful not to change
# the 'exec tail' line above.
menuentry "NixOS" {
        set root=(hd0,gpt3)
        configfile '/@nixos/boot/grub/grub.cfg'
}

menuentry "Arch" {
        set root=(hd0,gpt3)
        configfile '/@arch/boot/grub/grub.cfg'
}