This is for me the $1 million question I’ve never had a certain answer about.
I learnt long ago that in newer 32bit-64bit PCs the initial phase of the startup process is controlled by firmware and is later taken over by software (loaded from the Hard Drive or SSD into memory during boot) which controls the system from them on. The reason being that the software drivers will usually be more up-to-date and will run the system better than the initial firmware drivers necessary to boot the computer.
According to this, the POST and initial image on the screen is controlled by firmware (software in a motherboard chip rather than loaded into Memory from the hard drive).
My doubt is whether this is also the case in Macs.
This is important in troubleshooting because it helps to narrow down whether software or firmware may be causing an issue (such as the display glitching at the beginning of the startup but not at the end of the boot or afterwards, after the software has taken over, which would hint the firmware is at fault rather than the software) or "the screen glitching after boot can't be attributed to firmware as firmware is not running after boot".
My doubt is if something different may be happening: such as “the initial image on the screen is controlled by software, because the software has already taken over”, which I think is probably not the case.
I’d very much appreciate some wisdom on this.