All it required was the command bless, until that was undone by SIP. Since the T2’s secure boot, it has become steadily more complex, and now is only really possible from the internal SSD.
boot.efi
From the Macintosh ROM of Classic days, to Open Firmware in Power Macs, and on to (U)EFI with Intel, and ending up with LLB and iBoot in Apple silicon Macs.
One command, bless, used to do it all, copying boot.efi to the right place and declaring the volume bootable in its header. Where are we with Big Sur and M1 Macs?
What to do when your Mac won’t boot into the Desktop and Finder – another summary diagram for diagnosis and suggested solutions.
Adds details of the sequence of screens displayed at each stage, to help diagnose boot problems.
You want to boot your Mac from an external drive, but it doesn’t work. A diagram to explain what can go wrong, and how to fix it.
The command bless used to be able to control all things to do with booting, but has become steadily less useful with the advent of SIP and others.
Single-diagram summary of how a modern Mac starts up in macOS 10.12 or 10.13, from pressing the Power button to running the kernel.
Listing of known NVRAM variables and their usage/meaning, together with notes on how to change them.
What boot.efi does to prepare to load and run the kernel: building the hardware device tree, handling startup key commands, and more.
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