What Type of file system is used in EFI partition ?
EFI partition in Ubuntu is basically a FAT partition with label “EFI”
How to create / restore an efi partition ?
Just reformat or create a normal FAT file system partition with a size of 512MB (can be any size) and label it as “EFI”, You can use fdisk command if you are comfortable with cli or you can use gparted tool from a live media of Ubuntu to create new efi partition.
How can I reinstall Grub efi bootloader after creating or restoring efi partition ?
After restoring or formatting efi partition your system will not boot since bootloader needs to be reinstalled on the new efi partition, follow this tutorial to reinstall grub efi bootloader in your Ubuntu system.