Okay, IIRC, the article I read in the 80's said Moroder found some lost footage in the form of stills made from each frame of the film. (Which apparently was done as a form of copyright protection in olden days.) He reshot these back onto motion picture film the same way you would shoot animation cels. In other scenes, he animated single stills by moving the camera or adding movment like clouds over one shot of the city. As I think was mentioned in another thread, he recreated the tombstone of Hel, (from another still) and shot a new scene. The epitaph is in English, I presume it's German in the original lost footage.
Lucas talks about on one of the SE DVD's how he couldn't do Coruscant in the OT days. I immediately thought of Metropolis and chuckled. Even the 1930's Buck Rogers has a large cityscape with constant air traffic.