Boot sequence for USB mass storage

[bob prohaska]

Supposing for a moment that I want to use the bootcode.bin-only
method of booting a Pi3 can somebody tell me how much of the
software in the DOS partiton of the USB device is invoked?

I've always thought that u-boot on the microSD started u-boot
on the USB device, but it's never stated in so many words.

It's unclear to me if the steps:

Recurse through each MSD

If bootcode.bin found boot

mean _run_ the newly-found copy of bootcode.bin or merely
use it as a flag to denote which device to boot.

I've waded through the voluminous material at
https://www.raspberrypi.com/documentation/computers/raspberry-pi.html
and haven't found an answer that I can recognize. Apologies if I'm dense!

Thanks for reading,

bob prohaska