The relationship between states and localities is governed by the constitution and laws of each individual state and not the US Constitution.
In my particular state, and in many but not all others, local governments whether that be counties, cities or towns are administrative districts which only have the rights and powers which the state chooses to delegate to them through state law and the particular charter granted by the state to the administrative district. The state through the normal legislative process can change those rights and powers or even eliminate a particular administrative district.
