One of the interesting features on the BTTF Part 2 special features disc is the making of, where they show how they made the DeLorean fly. It was a combination of practical effects and special effects. For example, as the DeLorean was swooping in for a landing, it would be digitally composited into the film before being hidden by something in the foreground, such as a tree. They would then use camera tracking to re-do the exact same shot, only this time with the real DeLorean lowered from a rig at the angle they wanted as it appears on the other side of the tree. Then ILM went in and integrated the two scenes together.
A similar technique was used in the dinner scene, where Marty and his two kids were all played by Michael J Fox.