Does Unity Pro come with any more source code?

I want to make a physical sky/daylight system, (i.e. rendering with actual luminance and a volumetric sky) and I’m somewhat new to shader programming so I’m not exactly sure all that will be required - however I have seen that unity pro has more built-in rendering features enabled… how much of the code for that is available to change? For example is the hdr and shadow mapping code able to be modified in Unity Pro?

Unity Pro isn’t a separate thing; it’s just a license. If you downloaded Unity, you already have everything that it comes with.