In Airbus case, who have been doing full fly by wire for a while now, there are at least two completly seperate software implementations which run in parallel and cross compare the result. They also run on redundant flight computers with different hardware architectures.

Boeing probably has a similar thing for the fly by wire fighter jets they are involved in but there passenger planes are still mainly directly controlled by the pilot.

Some material: https://www.researchgate.net/publication/26587285_Challenges... https://www.researchgate.net/publication/220845884_Approache...