There are drone repair stations set up all over Earth, all manned by exact copies of Jack and Victoria. The Tet orbiting in space is in fact an alien craft that programmed the drones to kill all surviving humans on the planet. The Jack and Victoria clones were made when the aliens abducted the real Jack and Victoria years before invading the planet. Julia was ejected from the shuttle Jack was on moments before the aliens took him.
The Jack in the movie is about is one such clone. Jack, Julia and Beech plan to reprogram a drone to fly a nuke into the Tet, but the drone is destroyed in an attack on the human base, leaving them with no choice but to have Jack fly it up himself. Julia goes with him, but Jack puts her in cryo sleep and drops her off on his secret cabin on Earth and goes up to the Tet with Beech in her place. They detonate the nuke and destroy the Tet, disabling all the drones on Earth. Years later, Julia is at the cabin with her and Jack's daughter and one of the Jack clones arrives at the cabin. (Earlier in the movie, Jack encountered one of his clones who had memories of Julia and knocked him out).