Bootloader src/compile allows specifying a boot loader, I assume that is what is pushed initially to manage transfer on Maix side.
Is the boot loader run on RISC chip or co-processor?

I seem mention of open-ec, is that the same?

Is boot loader source available somewhere?

I remember bootload in kflash options is just program in ram, run on k210.
open-ec is the downloader firmware for stm32 on Maix Go