Suppose that we have a box: kind of box that might contain, for example, a cat. Suppose that no energy is flowing into or out of the box. And suppose we them ask: what is going on inside the box?
Sean Carroll will answer, in accord with all experimental evidence and of the most sophisticated quantum mechanical thinking, that nothing is going on inside the box:
The possible measurement outcomes... spinning clockwise or counterclockwise... only become “real”... when the quantum system interacts [outside] with a large number of degrees of freedom, becomes entangled with them, and decoherence occurs.... [But] what dynamic processes are occurring while the wave function isn’t changing at all? Your first guess here--nothing at all “happens” inside a wave function that doesn’t evolve with time--is completely correct....
Surprisingly, this claim--“nothing is happening if the quantum state isn’t changing with time”--manages to be controversial!