What happens when a quantum system is measured?

A measurement writes a record, and writing a record deletes every comparison between the possible answers. Picture a mail room. Letters keep their contents and go into labeled bins, and anything that only made sense between two bins is thrown away. Physicists call that deletion decoherence. What comes out is an answer that commutes with every other question the observer can ask, so it reads the same the second time and a neighbor who reads it gets the same result. That is what makes it a classical fact. Nothing gets disturbed, because no definite value was sitting there beforehand. The odds follow the Born rule, the only rule that adds up consistently across every way of asking. Nothing happens to an electron at the screen except that a record gets written, and the write cannot be undone.

Also in Chapter 28: What Is a Particle, What Is a Wave, and Why Does It Look Like Both?, Chapter 33: Why Was Every Open Problem the Same Problem?.