The Union is the collective of the 50 respective commonwealths united by and under authority of the Constitution, and the geographic area they occupy.
There is no provision of the Constitution that grants Congress power of territorial or personal legislation anywhere within the Union—only subject-matter legislation over certain things (Article 1 § 8(1-16)).
This means that Congress have no legislative power over property located anywhere within the Union or any American residing there, a limitation confirmed by the Supreme Court:
“The several States of the Union are not, it is true, in every respect independent, many of the right [sic] and powers which originally belonged to them being now vested in the government created by the Constitution. But, except as restrained and limited by that instrument, they possess and exercise the authority of independent States, and the principles of public law to which we have referred are applicable to them. One of these principles is that every State [of the Union] possesses exclusive jurisdiction and sovereignty over persons and property within its territory. . . .” [Underline emphasis added.] Pennoyer v. Neff, 95 U.S. 714, 722 (1878).