> read bytes from USB storage device plugged into a particular socket inside the box and branch to them
Well, they have first to recognise that the USB disk first sector contains executable code, else they will often jump to a lot of zero instructions.
How some do it is a little bit boring; in my experience with Gujin few BIOS will check that the Windows MBR is present by checking bytes at offset 0x0c to be identical to the Windows MBR ones.
IHMO checking the first byte would be better (either jmp or cli) when the 0xAA55 signature is present.
But what I wanted to say is that at the bootloader level you do not have USB, you have OHCI, UHCI, EHCI or xHCI - and more in the embedded world.
The complexity for the boot sequence is that you may be on a BIOS/EFI which only handle a previous generation (only USB1 on USB2 hardware, only USB2 on USB3 PCIe card) so you know where (PCI address) is the device you are booting from, but in Linux this PCI address do not exists because the full USB chipset is supported.
Handling these corner cases (as I tried in Gujin) is not really easy.