Amendment X. The powers not delegated to the United states by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.
Separate from the federal system, each state has its own legal system. All 50 states have their own constitution. A state constitution may be interpreted independently from, but consistently with the federal constitution. Each state also has its own statutes that have been codified into codes.
Each state has its own court system with trial courts and appellate courts and judicial precedents derived from them. Likewise, the states have their own executive and their own administrative agencies that create rules and regulations that are binding on the residents of that state.
An issue that occasionally arises is who has authority over a particular area of law: the federal or state government. First, the federal government only has power to act if that power was given in the U.S. Constitution. However, language such as "necessary and proper" has (Art. II, sec. 8, cl.18) been interpreted broadly in favor of federal legislative power. If the federal government (essentially Congress) has authority and has exercised its authority to act, the resolution is based on whether Congress in acting intended to preempt state action. If so, that intent controls and the federal government possesses exclusive power in that area. If not, states have concurrent power to enact additional laws as long as they are consistent with federal law.