On September 24, 1789, U.S. Congress passed the Judiciary Act of 1789 to establish a 3-tiered Federal judicial structure topped by a Supreme Court.
You may have heard about "court packing" (increasing Supreme Court's size) since Justice Ginsburg's passing. You may have wondered, how is that possible?
The answer is that, Article III of U.S. Constitution, which establishes the judicial branch -- the third branch of U.S. government -- is actually pretty short.
Section 1 of Article III simply provides: "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. The Judges, both of the supreme and inferior Courts, shall hold their Offices during good Behaviour, and shall, at stated Times, receive for their Services, a Compensation, which shall not be diminished during their Continuance in Office."
Section 2 of Article III talks about what types of cases fall under this general jurisdiction; and Section 3 talks specifically about cases of "Treason against the United States."
Beyond that, it's up to Congress and State legislatures to set rules for Federal and State courts, respectively.