What document explains how power is shared between national and state government?

The document that explains how power is shared between national and state governments in the United States is the U.S. Constitution. Specifically, the Tenth Amendment delineates that powers not delegated to the federal government nor prohibited to the states are reserved for the states. Additionally, the Constitution's Supremacy Clause establishes that federal law takes precedence over state law when there is a conflict. This framework creates a system of federalism, balancing authority between national and state governments.