IIRC the people who programmed the first auto pilot for a major airliner were required to be on board the first test flight, so I have to think their testing methodology was pretty meticulous.
Some airlines do not allow pilots to manually fly above 3k feet, nor allow first mates to land manually[0].
0: https://www.wired.com/story/boeing-autonomous-plane-autopilo...
There is a (prevalent) happy path where autopilot and autoland can get you the whole illusion "but the plane flies itself" - but the whole system is built around humans catching and handling all and any exceptions.
A USAF C-54 made a complete transatlantic flight (takeoff and landing included) on autopilot back in 1947: https://www.flightglobal.com/pdfarchive/view/1947/1947%20-%2...
Modern Cat IIIc (zero-zero) autopilots handle everything except taxiing and takeoff. And I think the takeoff thing is more political than technical.