yes, afaik, it’s a UEFI bootloader, probably stored in firmware. I don’t have a strong grasp of the Pi’s bootsequence yet, but presumably it knows to look for boot partitions on the microSD (mmcblk0) and on later hardware, /dev/sd*.
I came across this the other day while researching but haven’t had a chance to dig into it: GitHub - raspberrypi/firmware: This repository contains pre-compiled binaries of the current Raspberry Pi kernel and modules, userspace libraries, and bootloader/GPU firmware..