Two of (arguably the best) open source RC aircraft flight controllers (ArduPilot and PX4) are using extended Kalman filters in their state estimators (essentially sensor fusion that provides attitude/position estimate):
https://github.com/ArduPilot/ardupilot/tree/master/libraries...
https://github.com/PX4/PX4-Autopilot/blob/main/src/modules/e...
I'm not that familiar with cleanflight/betaflight/inav scene to know what the FPV racer flight controllers use.