There are two basic approaches one can take to thinking about the United States Constitution. One is to treat it is a given, whether that refers only to the specific document proposed and ratified in 1787-88 with subsequent textual amendments or to the body of case law developed over the past two centuries. In any event, the lawyerly task with regard to a constitution that is treated as already existing is to engage in debates about its interpretation.
What, for example, are the powers of the President? One might focus on the difference in language between Article One (“all powers herein granted”) and the more laconic language of Article II (“the executive power”) or on the meaning of the Commander-in-Chief Clause; or, commonly, one might parse at length various opinions issued by the Supreme Court over the years, including, of course, Justice Jackson’s now-canonical opinion in the Steel Seizure Case, with its “three-part test” elaborating the basic structure of presidential authority. There are, of course, literally thousands of other possible examples.