Each example has its own state that you can access at the state variable and change with the setState function. Default state is {}.
<div>
<button onClick={() => setState({isOpen: true})}>Open</button>
<Modal isOpen={state.isOpen}>
<h1>Hallo!</h1>
<button onClick={() => setState({isOpen: false})}>Close</button>
</Modal>
</div>If you want to set the default state you can do something like that:
initialState = {count: 1};
<button onClick={() => setState({count: state.count+1})}>{state.count}</button>