He also has a fun YouTube channel: https://m.youtube.com/playlist?list=PLTSCOv-lGtMax-oA4Pnq8OT...

One of the first flight controllers he coded was in Arduino..

https://github.com/nickrehm/dRehmFlight/tree/master/Versions...

He deliberately kept the code in single file and made sure it was still easy to follow.