I want to create a Custom kernal (for the Raspberry Pi). From Scratch. Where do I start?

I see tons of tutorials explaining how to edit configs and then build the kernal for the pi, but they are all based off the Raspbian linux kernal.

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I want to be absolutely ridiculous and design a 𝒇𝒓𝒆𝒔𝒉 new kernal. I figured the easiest way to begin would be with a Raspberry Pi and then move up from there.

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So I am asking, where would I begin searching for information on this endeavor?

Is this even possible due to closed source architectures or other reasons?