The efi-update-loader script looks for efi type partitions on each disk associated with the root filesystem, looking at UFS and ZFS mirror, stripe, raid etc. configurations.
Unless the user specifies a device to use, it will use all such ESPs, and copy /boot/loader.efi (or the otherwise
specified loader file) to /EFI/FreeBSD/loader.efi and backup any existing file as loader-old.efi if the new loader.efi isn't the same as the old one.
If there's a FreeBSD loader/boot1 as /EFI/BOOT/BOOT{arch}.efi it will move it to /EFI/FreeBSD/BOOT{arch}-old.efi.
If there's no Boot Manager entry for FreeBSD, it will warn the user but not run efibootmgr automatically.