When I was around 11, our local grocer who liked kids, would let me exchange comic books as long as I kept them mint. Superboy and Superman were my favorites. I would always fantasize about being able to fly.
I can recall the miniature city but not how it came to exist.
Brainiac’s debut had him coming to Earth and shrinking down cities (including Metropolis) in the same manner that the Kryptonian capital city (”Kandor”) was shrunken down. The character’s original intent for the cities was to use them somehow in restoring his home planet, of which he was ruler/despot.
Funny enough, Superman didn’t fly originally in the comics; he leaped great distances, similar to what the Hulk does. The original explanation for the leaping ability was due to Earth having a lower gravitational pull than Krypton; the principle kind of foreshadowed how astronauts were able to jump over eight feet on the moon even while encumbered with a spacesuit.