This section of the guide will eventually provide you with a short guide to build your own Custom Elements that will act as reusable components in your application.
If you can't wait, the tl;dr is that Lit-Element is a fantastic starting point for this. Go nuts!
But don't forget that you can also use three-elements with any other frontend framework that modifies the DOM (eg. React, Vue, Svelte. I don't know, are there any others? I'm just kidding. THERE ARE MILLIONS. AAAAaaaaAAAHHHh)