On a CoreStorage Filevault 2 system, the Recovery HD is used as the boot loader, calling an EFI program named "boot.efi" present on the filesystem.
On a APFS system things are a bit different; the Recovery HD is still used, however this is now a Logical volume presented from the main volume group, with the update to High Sierra a Firmware upgrade was pushed out to all supported systems enabling the EFI to grok APFS.
