I've read https://help.ubuntu.com/community/UEFI but I'm still puzzled with the flow of boot process in UEFI mode for Ubuntu.
I understand that the first step is UEFI firmware will load bootx64.efi provided by Ubuntu which is stored in ESP. What this file does? Launching GRUB2? Is it safe to just copy bootx64.efi from one disk to ESP in another disk?