Reason you use 1 1/2" PVC and 2" ID discharge hose is due to wall thickness of schedule 40 putting the pipe at about 1.90" O.D. simple hose clamp to secure the two together and so you can break it down set it up easier for transport.
The assorted fittings are situation dependent to adapt to your personal compressors or rentals. If you are going to use it while diving scuba or something similar please be safe and use a 1/4 turn ball valve near the inlet point so you can shut it down if it tries to get away from you and doesn't bring you up uncontrollably.
Can always use different sized reducers like a 1 1/2-4" etc or maybe even some sort of roof flashing for around pipes to create a vacuum cleaner type attachment point on the "Business end".
