Boeing does not have a box of parts like that! They send them to assembly and repurpose them on new aircraft. Don't worry, aircraft break on a regular basis, so the sum-total of parts remains constant. For every part that gets left off, a bit of bailing wire is added to hold something else in place. I speak from experience. I spent 21 of the last 27 hours on a Boeing KC-135 built in 1959 and some of that (14 hours) was flight time.
"My KLR belongs to God, when he wants it clean he washes it."