Most server side controls have logic that gets executed in the CodeBehind page for that particular webform. If you take the ID (that is how the CodeBehind knows which item to set properties for) and search in CheckoutShipping.aspx.cs, you should find all of the underlying logic for that particular control. Eg:
Code:
CartSummary.Text = cart.DisplaySummary(true, false, false, false, false);
So as you can see, the text output for the CartSummary literal control is generated by the DisplaySummary method of the ShoppingCart object. If you open the solution in visual studio, you can right click DisplaySummary and "Go To Definition" to go right to the method in the ShoppingCart class. You'll need full source and "real" visual studio though... Express versions do not tend to do the trick. Just basic object-oriented programming