Complex sentence: Magneto used the skyscrapers that the mutants were cornered in alongside the Chrysler Building, the Empire State Building, and Grand Central Station to create it by controlling the iron works in the buildings to remove them from their usual locations and reconstruct them into one large building.
Split into simpler sentences: Magneto used the skyscrapers that the mutants were cornered in alongside the Chrysler Building, the Empire State Building, and Grand Central Station to create it. He did this by controlling the iron works in the buildings to remove them from their usual locations and reconstruct them into one large building.