To add to this, most of the time one doesn't need to hack on the drone firmware itself. Just configuring/hacking the mission planner alone would go a long way: https://ardupilot.org/planner/

But if you're interested in more readable codebases without too much legacy headaches:

https://github.com/BossHobby/QUICKSILVER (for indoor drones mostly)

And for even less functionality but a good learning tool kind of codebase:

https://github.com/nickrehm/dRehmFlight (Do check out his YouTube channel.. he does some cool stuff)