https://mattkprovideo.com/2017/04/04/ok-i-think-i-know-how-they-did-it/

How they showed Superman flying away and in the same shot, Clark Kent comes in the door.

OK, I think I know how they did it….

look at 3.51 in this youtube clip, we see over Lois’s shoulder Superman saying goodnight and flying away. Then in the same shot, NO cuts she walks over to the apartment door and lets Clark Ken in… same shot.  How they did it?

Had a Superman double in the shot? No, thats clearly Christopher Reeve as Superman in the same scene as Christopher Reeve as Clark Kent.  Did they have Reeve get pulled by wires offscreen and he quickly changed out of his Superman suit and into Clark Kent clothes?  Possibly but highly unlikely…. Superman could move that fast, Christopher Reeve couldn’t.    A hidden jump cut? I’ve rewatched it studied that shot and theres no cut.  Today they could hide a cut with a digital morph, you couldn’t do that in 1978.

No, look at 3.51 again. Margot Kidder is standing  in front of a PROJECTION of (earlier shot footage of) Christopher Reeve flying away then she walks away from the rear projection scene onto a practical set and then Christopher Reeve as Clark Kent walks in. All done IN CAMERA.