Code documentation for students

Hello everyone,
I’m an engineer student and I’d like to implement some feature in cycles for academic purpose.
I’d like to work in the frequency domain to try implement some filter in rendering instead of post processing.

I’m looking for Code and Documentation about cycles, but i can’t find anything enough precise anywhere. I tried exploring the Git but after a couple hours i gave up.

Can anyone show me where to find what i’m looking for?

Thank you all