Help with understanding the HITL environment

I have been trying to go through the documentation to understand how to use the HITL modes. I am particularly interested in the X-Plane one:

Can someone please help me check if what I know/understand is accurate? There are 5 modules illustrated in the above image:

  1. PX4 HITL : This one is my Pixhawk controller.
  2. QGroundControl: This is the QGC software we use as the ground control software and to update/configure the flight controller.
  3. Joystick: I have previously cofigured my Futaba RC Transmitter to work with QGC. Is this module meant to represent that?
  4. API/Offboard: No idea what this is! What is it?
  5. X-Plane Simulator: This is my installation of the X-Plane 10 that I will be using to simulate the drone.

Also, a bonus question I have is about Mavlink. Does it come installed with QGC? Or is it something I have to manually install?