Bootloader is built somewhat similarly to the Ubuntu system, but is kept separately (the bootloader is never changed during system upgrades).
The grub-mkimage utility is used to build both the EFI first stage loader (built as /boot/efi/EFI/Boot/bootx64.efi, to be placed as /EFI/Boot/bootx64.efi on the efi partition) as well as grub loadable modules (built in /boot/grub/x86_64-efi, to be placed in under /x86_64-efi on the grub partition).
After this is built, the sub-directories /boot/efi and /boot/grub are used to build VFAT filesystem images to go into the first two partitions of the final disk image.