In the explorer, texture layers are located under the materials and shaders to which they belong. The easiest way to explore texture layers is to set the explorer scope to Materials.
You can expand a material to access its texture layers, and you can expand the material's shaders to access their texture layers. All layers belonging to a given material or shader are under the TextureLayers node.
When you expand a given texture layer's node, you can see which shaders are connected to its ports. You can also see which of the parent shader's ports have been added the texture layer.
The image on the left shows a material's hierarchy expanded to show the texture layers added to its Lambert surface shader. The image on the right shows the same thing but for the material itself rather than one of its shaders.
