When you do.

npm start

index.html will be served in web browser.

Now, index.html contains root dom node and as it served the control goes to it's javascript file which is index.js.

Now, the index.js having below rander function.

ReactDOM.render(
  <React.StrictMode>
    <App />
  </React.StrictMode>,
  document.getElementById('root')
);

As you can see above <App /> is rendered.

Now, <App /> component exported from App.js file.

So, this how we do changes in App.js and it reflects on web browser.