How is software verifiable whatsoever for correctness? Ironically hardware is I mean pure functions maybe verifiable for correctness, but I don't know how you

How is software verifiable whatsoever for correctness? Ironically hardware is I mean pure functions maybe verifiable for correctness, but I don't know how you verify facebooks codebase is 'correct' Maybe try to run a maximizing function on the stock value, very unfeisable ofcourse