Presumably, nothing was in superposition when the universe began. Everything after that evolved in time according to the Schrodinger equation, with everything b

Presumably, nothing was in superposition when the universe began. Everything after that evolved in time according to the Schrodinger equation, with everything becoming more superposed (i.e. their exact properties less certain) and entangled with everything else to at least some degree. Things can be more or less entangled with other things, and when you measure something you become very strongly entangled with it.