multithreading - What are the differences between event-driven and thread-based server system?

• Node.js is an event driven I/O and It's a single threaded server that
  acts upon callbacks and never blocks on the main thread.

  
  
  
  
  
  1. But how does it manage to non-blocking I/O?
  
  
  2. if it does easy to manage, why don't thread-based system manage it?
  
  
  3. Does not work the other threads (behind single event-driven thread) as like thread-based ?
  
  
  4. if the other threads mean workers(behind event driven thread) are busy, how it still can handle jobs without blocking?
  
  
  



• Thread-based model assigning a task to a thread and if there is no
  idle thread, block new tasks.

  
  
  
  
  
  1. if a thread can handle multiple tasks like as event-driven single
     thread that handles every I/O without blocking, why thread-based
     system doesn't use this tactic on busy threads to I/O without
     blocking.
  
  

I am wondering what are the differences (advantages/disadvantages) between event-driven and thread-based server systems.