How does the front end connect to the back end in the code editor?

Apologies if it’s a silly question but I did a tutorial on setting up an email application where you type in your name and it spits out a password for you to use and an email it generates for you.

I did this in the code editor and it showed me the results in the terminal.

But I don’t understand how it gets from that terminal, onto a website with a pretty box that pops up and a page that lets you enter in the details instead of me doing it on the editor. How do I get to the editor to that and what does that involve?

Is it me somehow linking through maybe using “import” to other bits of code that are JavaScript, CSS and HTML?