When you do.
<pre><code class="lang-javascript">npm start
</code></pre>
<code>index.html</code> will be served in web browser.
Now, <code>index.html</code> contains root dom node and as it served the control goes to it's javascript file which is <code>index.js</code>.
Now, the <code>index.js</code> having below rander function.
<pre><code>ReactDOM.render(
 &lt;React.StrictMode&gt;
 &lt;App /&gt;
 &lt;/React.StrictMode&gt;,
 document.getElementById('root')
);
</code></pre>As you can see above <code>&lt;App /&gt;</code> is rendered.
Now, <code>&lt;App /&gt;</code> component exported from <code>App.js</code> file.
So, this how we do changes in <code>App.js</code> and it reflects on web browser.

When you do.
```javascript
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.

How React serves?