How different are these planes from each other!?

Update: I used to think when a plane company produce a certain plane, it is only that version of the plane. They may make some modifications and improved on it, but it is still basically the same plane.
Update 2: Hence, I am very surprised Boeing has a number of versions of the 737. Likewise, it also has a number of versions of the 747 and 777. Likewise, Airbus also has a number of versions of their A350s and A330s.

What is so different among these planes? They are not just minor upgrades to the planes, are they?
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