As said here the boot process:

GPU Core
first stage bootloader, which is stored in ROM on the SoC
bootcode.bin (loader.bin merged)
start.elf
config.txt
cmdline.txt
kernel.img

After this boot scripts will be called by the kernel: systemd or init

Raspbian

After the failure of the Commission project, 400 000 Euro was given to the CleanIT initiative in order to address one of the three issues – terrorism. The suggestion was that unspecified technology companies could take action against unspecified problems in order to address undefined “terrorist use of the Internet”. Unsurprisingly, due to the lack of focus of the project, the first year of “work” had only produced a 23-page list of sometimes extreme, sometimes illegal and always unfocussed suggestions of things that could, theoretically, be done to address the unspecified problems the project was set up to solve.