Just like with systems reliability, nobody knows it well. But some things we do know are that keeping it simple quickly becomes more impactful than adding failsafes, and you need to design quality holistically from the top down, and not focus at failures or known failure modes.

And just to say, the fact that nobody has any idea how to engineer systems reliability is the main reason why we have a duopoly in large airplanes that everybody expects to turn into a monopoly soon. If we knew how to do it, companies would pop into the market all the time.