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The source code of the built-in shaders is typically hidden in Unity. You cannot open this from the editor as you do with your own shaders. For more information on where to find a large portion of the built-in Cg functions for Unity, go to your Unity install directory and navigate to the Editor | Data | CGIncludes folder:

In this folder, you can find the source code of the shaders shipped with Unity. Over time, they have changed a lot; you can visit the Unity download archive (https://unity3d.com/get-unity/download/archive) if you need to access the source codes of a shader used in a different version of Unity. After choosing the right version, select Built in shaders from the drop-down list, as shown in the following screenshot:

There are three files that are of note at this point: UnityCG.cginc,  Lighting.cginc, and UnityShaderVariables.cginc. Our current shader is making use of all these files at the moment. In Chapter 11, Advanced Shading Techniques, we will explore in depth how to use CGInclude for a modular approach to shader coding.