Internals
Where to find the source code and talks about how the engine works.
Source code
FollyChess's source code is hosted on GitHub at github.com/aryann/follychess.
Talks
The engine has been the subject of two talks, both titled From 20 Nanoseconds to One: Optimizing Bishop, Rook, and Queen Move Generation in a Chess Engine. They cover how FollyChess generates sliding-piece moves and how that code was optimized: