Main question is resolved.GRUB knows how to reach grub.cfg because grubx64.efi binary has the UUID and path to grub configurations.
1. sudo blkid; 2. sudo bash 3. cd /boot/efi/EFI/Mangaro 4. strings grubx64.efi 5. And at the output of strings there is UUID and /boot/grub. Summary: GRUB installation not only involves configuration of text files, but also it involves generating data in binary grubx64.efi.