Do I need to install HTML CSS and js on the ubuntu server to host an HTML CSS and js website?
You can’t install any of those. The browser is built to use them. As long as the HTML is served and it links to the CSS and JS the browser will do the rest. What you need is a web server to serve the page.
How/where is Ubuntu running?
There are a lot of much simpler options than hosting and managing your own server. Although it might be worth it just for the practice. I would however not really suggest it as the main way of getting web pages on the internet.
If you need JS server side (that is executing the JS code on the server) then you need something like Node.js, Deno.js, Bun.js, or whatever.