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 ...
