Good question, Ari.
Article III, Section I says that "The judicial power of the United States, shall be vested in one Supreme Court, and in
such inferior courts as the Congress may from time to time ordain and establish." (Emphasis added.) Jurisdiction stripping proponents claim that since the lower federal courts (U.S. district courts and circuit courts of appeals) depend on Congress for their very existence, Congress is free to limit their jurisdiction. A highly dubious conclusion, IMO, but there it is.
As for the Supreme Court,
Article III, Section 2 provides that "the Supreme Court shall have appellate jurisdiction, both as to law and fact, with
such exceptions, and under such regulations as the Congress shall make." (Emphasis added.) Jurisdiction strippers see this provision as an absolute grant of authority for Congress to limit the Court's appellate jurisdiction. The scope Congress's authority under the Exceptions and Regulations Clause is "unsettled," to put it mildly.