ArduPilot exists. And was used in the attacks on russian air bases https://www.reddit.com/r/ardupilot/comments/1l0pdg9/sbu_appa...

The hardware typically runs https://pixhawk.org/ on 3d printed airframes https://diydrones.com/profiles/blogs/ukraine-s-drone-air-for... or to OP's point, DJI drones flashed with custom firmware