They have an override, and it is called a Pilot. When things go haywire, the first thing well trained pilots are taught is to disconnect and manually fly the plane.
Correct. This issue is sometimes confused because all modern large airplanes use electronic controls between the pilot and the control surfaces, such as the tail. This is called "fly by wire." Decades ago this was done mechanically, sort of like power steering on an automobile. Nowadays there is no direct, mechanical link between pilot's controls and the surfaces. That is safer, because there are multiple pathways from the controls back to the tail, for example, whereas with the older systems there were only two links (as I recall). In an accident, both links could be severed.
Anyway, the autopilot can be disabled by the pilot, but the fly-by-wire system cannot. Both are electronic. I doubt anyone could hack either system.
Fly-by-wire is MUCH simpler, as you see from Figs. 3 and 4 here, a document from 1968 when fly-by-wire was first being developed:
https://apps.dtic.mil/sti/pdfs/AD0679158.pdf
Figure 4 says "NON REDUNDANT" but they are all redundant nowadays. As I said, more redundant than the last mechanical systems that fly-by-wire eventually replaced.
Concerning cars, the legislation doesn't allow a 100% electric link between the steering wheel vs wheels, even if the rack is hydraulically assisted then now electrically.
