title: where is process.env located
body: <p>I'm learning React and React-Redux and I'm trying to edit an existing project I found.</p>
  <p>Inside the <code>reducers</code> folder there is a file that has this line of code:</p>
  <pre><code>const { data } = await client(`${process.env.REACT_APP_BE}/videos`);
  </code></pre>
  <p>But there was no declaration of <code>process.env</code> . Where can I find that file?</p>