check valve prevents the back flow of relatively high pressurized liquid to the pump in the discharge line so that the impellers stay safe .

مع اني لم أشاهد الفيديو حتي الآن ولكن محبس عدم الرجوع يحمي المضخة من الهامر وذلك بمنع رجوع هذة الصدمات إليه في حالة توقف الحركة فجاءة او إنقطاع الكهرباء وبذلك يمنع رجوع هذة الصدمة الي ريشة المضخة وشكراً

At 2:40 it could/should be upside down in my opinion.

The purpose of the eccentric reducer for being upside down is to prevent trapped air. Lets assume you put the reducer slope section up, now put a straight line following the upstream flow pipe diameter. That triangle space above the reducer will trap air and other gasses and creates bubbles in the liquid. You are pretty much disturbing the flow. On a centrifugal pump, the last thing you want to have is bubbles on the suction side eye socket. Those bubbles will burst on the impeller vains with supersonic velocities causing damage to the impeller. This is called cavitation.

So the fact that the pipe is at an upward angle doesn't let air escape? Well.. I guess not if there is nowhere for the air to escape to x) unless there is a bleeder valve of some sort I suppose.

You always assume even with the incline the pipe is full of liquid all the way to the inside diameter wall. Bleeder valves are typically used if you are dealing with hot liquids and heatexchanger piping. Honestly I don't see any reason not to use a bleeder valve on the suction side of any pump piping.