In this module (and also in the next module), we always work on one *.js file only and we only include (link) one *.js file at the same time to our HTML file.
That's not a must-do though!
If you build more complex websites, that might required different scripts that do different things, you can absolutely include multiple, separate script files into one and the same *.html file.
You can simply add multiple <script src="..."> elements into your HTML document:
<head>
<script src="first-script.js"></script>
<script src="second-script.js"></script>
</head>
</body>
...
<script src="third-script.js"></script>
</body>The linked scripts will be parsed and executed in the order you defined in the HTML file. It also doesn't matter whether you link them in your <head> section or your <body> section (or if you do both, as shown in this example).
You can also define functions or variable in file A and use them in file B, though you should make sure that the file where you define them is included (linked) before the file where you use them.
So in the example above, we could define a variable in first-script.js:
let myName = 'Max';
and then use it in second-script.js:
console.log(myName);