Superman didn’t become Superman. Superman was born Superman. When Superman wakes up in the morning, he’s Superman. His alter ego is Clark Kent. His outfit with the big red “S”, that’s the blanket he was wrapped in as a baby when the Kents found him. Those are his clothes.
Flash and I are wearing the same Superman outfit from Bleh. Free Superman pose from same shop and in fact everything else is free there, superheroes eh!