The Joint Chiefs of Staff (JCS) is the body of the most senior uniformed leaders within the United States Department of Defense, that advises the president of the United States, the secretary of defense, the Homeland Security Council and the National Security Council on military matters.
The composition of the Joint Chiefs of Staff is defined by statute and consists of a chairman (CJCS), a vice chairman (VJCS), the service chiefs of the Army, Marine Corps, Navy, Air Force, Space Force, and the chief of the National Guard Bureau. Each of the individual service chiefs, outside their JCS obligations, work directly under the secretaries of their respective military departments, e.g. the secretary of the Army, the secretary of the Navy, and the secretary of the Air Force.
https://en.wikipedia.org/wiki/Joint_Chiefs_of_Staff
10 USC Ch. 5: JOINT CHIEFS OF STAFF
§151. Joint Chiefs of Staff: composition; functions
(a) Composition.—There are in the Department of Defense the Joint Chiefs of Staff, headed by the Chairman of the Joint Chiefs of Staff. The Joint Chiefs of Staff consist of the following:
(1) The Chairman.
(2) The Vice Chairman.
(3) The Chief of Staff of the Army.
(4) The Chief of Naval Operations.
(5) The Chief of Staff of the Air Force.
(6) The Commandant of the Marine Corps.
(7) The Chief of the National Guard Bureau.
(b) Function as Military Advisers.—(1) The Chairman of the Joint Chiefs of Staff is the principal military adviser to the President, the National Security Council, the Homeland Security Council, and the Secretary of Defense.
(2) The other members of the Joint Chiefs of Staff are military advisers to the President, the National Security Council, the Homeland Security Council, and the Secretary of Defense as specified in subsection (d).
(c) Consultation by Chairman.—(1) In carrying out his functions, duties, and responsibilities, the Chairman shall, as necessary, consult with and seek the advice of—
(A) the other members of the Joint Chiefs of Staff; and
(B) the commanders of the unified and specified combatant commands.