Scheme is the basis of basic forms.
Many origami are based on the same basic shape. In other words, at the initial stage, these figures add up the same way. So out of shape Solitaire (triangle) are added such basic shapes like kite, fish. Books are obtained from the house door, Catamaran.
Pancake originates from the basic forms book and Solitaire. Bird and frog are based on the basic form of double square. But frogs can also be obtained from the double triangle. Some of the basic forms are obtained by a combination of other basic forms.
Since fish is composed by putting the kite on it though. Below is a diagram showing what the basic form from which you can get.