Neutering a cat does not calm them down. Neutering can make your cat more affectionate and friendly towards strangers. It also makes them less likely to mark their territory – a behavior usually associated with fear or anxiety about people (they might be trying to protect themselves).
This isn’t to say that all neutered cats are calmer than unneutered ones – there are still plenty of cats who have been fixed in the past, showing signs of aggression when they’re around other animals or humans! But on average, it seems like neutering helps calm some behaviors down.