Willy,
For my limited understanding.
I don’t have much detail reading on the sdbuild for my case, unless I had some custom driver or communication needed I will not investigate much.
But I do had a little experience on custom PetaLinux + Ubuntu ARM build (Not PYNQ).
So what I understand is both PYNQ and custom build are so similar control and setup are all following the same backend settings (It will not become other build tool as all relying PetaLinux so far).
Now back to the question:
FSBL - my experience told me if you had a complete standalone ZYNQ design (No OS) and you are going to boot from other location i.e. DDR /NAND the first thing you would need is to tell the ARM to use bar bar bar so FSBL=First Stage Bootloader I will think as very first stage of BIOS? (Maybe I am wrong).
After that it will start setup more things and seek boot bar bar bar.
Things getting so complex maybe open a new post for other to answer this more deeply.
But as an user + developer + engineer why spend so much time to understand all…llllllll details when the top idea could support you enough to develop.