Missing The Airport?

I need to ask one my pilot acquaintances about this but I was wondering how the Northwest Airline pilots were able to overfly the airport even if they were busy with something else or even sleeping. I was under the, apparently false, impression that almost all modern passenger aircraft pretty much flew airport to airport on 'autopilot' including the post climb out and initial descent into the destination airport. One would think that the autopilot would have at least initiated the descent into MSP and alerted the pilots of the fact. Must be I'm misinformed about the the role the autopilot plays today.