In the Blink of an Eye

The part of the reading that really stuck out to me was the fact that when Apocalypse Now was made, 1,250,000 feet of film was printed, but we only get too see a very small portion of it. I'm interested to know what criteria the editors had to determine what footage made the cut. Even film productions that shoot a lot less footage have a large ratio between seen and unseen footage. I understand the desire to shoot as much as possible in order to find the perfect take, but how does a filmmaker know when they've found it? How does an editor know? What if they are unable to find it? What happens to all the unseen footage? Is shooting tons of footage actually helpful or efficient?
Also I never really thought about how accustomed we are to cuts in movies. I don't even think about them when they occur. I totally accept that as a logical reality when I watch a movie.