where federal and state laws, why does the federal law supersede state law to control the covered subject?

"This Constitution, and the Laws of the United States which shall be made in Purusance thereof; ... shall be the supreme Law of the Land; and the Judges in every State shall be bound thereby...

(U.S. Constitution, Article 6, Section 2)

Any law made by Congress is a law made 'in pursuance thereof,' since it is made with authority derived from the Constitution. All federal laws are made with Congressional authority or with authority otherwise derived from the Constitution, so those laws are supreme over any other laws in the United States, except the Constitution.

Where there are conflicts of law, the U.S. Constitution is the Supreme Law of the Land because it was created first by the sovereign people.

Article VI, United States Constitution

"This Constitution, and the Laws of the United States which shall be made in Pursuance thereof; and all Treaties made, or which shall be made, under the Authority of the United States, shall be the supreme Law of the Land; and the Judges in every State shall be bound thereby, any Thing in the Constitution or Laws of any state to the Contrary notwithstanding."

Best wishes!

It's hierarchical. The federal government controls subjects that are mentioned in the Constitution. All other powers are reserved for the states. See the Tenth Amendment of the Constitution.

"The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved for the States respectively, or to the people."

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The Rules Enabling Act.