By default, the Group Output node from Blender Geometry Nodes node tree, has two inputs. The first one, with index 0, is the base input for the geometry. The second input, with index 1, is virtual, used to create new inputs – when we connect a link to it, a new input of a certain type is automatically created on its place.
Creating procedural pixel shaders material in Blender by Ryan King Art.
Creating procedural buffalo plaid fabric material in Blender by Ryan King Art.