The process for Apple hardware to find and start EFI is described in the "Booting from an Apple File System Partition" section on page 22 of this document. Basically, the EFI image is read from a special block on disk.

https://developer.apple.com/support/downloads/Apple-File-Sys...

The boot process for HFS+ is more involved, because you have to actually read the HFS+ file system to locate the EFI image, since it's stored in a regular file.

I'm not sure what happens after jumping into the EFI code.

(Disclaimer: I worked on the linked document.)