When you call setState on a component, only that component's subtree is reconciled and re-rendered.
The React API will also let a component decide for itself if it should be rendered.
State in React is local to a component, but it can be passed down to a component's children in the form of props.
Centralized state can be achieved using a library like Redux, where an individual component subscribes to an external data store's changes. Redux abstracts the details away, but under the hood it's still just setState.