For example you
- use 1 efi-partition
- install ubuntu (extra partition, and ubuntu-version of grub to efi-partition)
- install XXXXX (extra partition, and XXXXX-version of grub to efi-partition)
- install manjaro (extra partiton, and manjaro-version of grub to efi-partition)
Evrey install will overwrite the previous “grub”.efi with its inserted path to the grub.cfg of this install.
At last manjaro will install its grub. This grub will point to the grub directory in the manjaro partition, where the modules for this grub reside, and where the grub.cfg of manjaro is.
These will be used. (Until you reinstall grub from elsewhere)
But there are /boot/grub directories in every partition. When running ubuntu you will see the /boot/grub/grub.cfg of ubuntu (which is not the one, “grub”.efi will use at boottime) 