This is what a computer is. This is what a corporation is. This is what the United States are, more or less, from when Z lives. This is what a credit card and a bank account are, and what advertising is, and what the police are, and all sorts of other things. This is not the emotion about them, just the information.
This is what a sim is. People use computers to have experiences that aren't happening in real life – here are some memories of that happening. But fake people are hard to make, so when nobody wants to be something complicated in a sim people use a Cranium.
A Cranium is the kind of squishy pod that Z was in. People take brains out, and put them in the Cranium, and then the person only lives in sims and not in a real body anymore. Lots of people use Crania to stay alive after they would have died, and take jobs being people in sims to pay for electricity and brain-food. They don't have as many rights as a person in a body, but they have a lot.
Sometimes, if you commit crimes, you can avoid dying or being in torture-prison by letting someone take your brain out and keep it in a Cranium. A brain with no documents in a Cranium doesn't have any rights at all. Z was one of those.