To create a geodesic dome . . .

. . . each of the basic triangles in a regular geometric solid is broken down into smaller segments.

Here is a typical breakdown of an icosahedral dome, with a berakdown frequency of 4. That means each side of the triangle is broken up into 4 separate segments. Each of these segments will be a strut, connected at each end to a vertex. The Breakdown Triangle Report provides details about which struts and verticies are needed to form a broken-down triangle that is part of the dome. The actual breakdown triangle, when built, does not lie flat. Rather, it takes the shape of the dome quite naturally because of the different sizes of the struts: