Java as a language is typically seen in the "back end" of websites, usually with data management, but it's not exclusively Java. Other languages include Ruby, Python, C#, or really any logic-based languages could be used.
If you're interested in coding websites, there's an absolute BOATLOAD of resources you can use to learn. But be warned, it's a whole discipline in and of itself, and you could very easily lose yourself for hours studying.
I actually have some websites published on my own Github.
As far as domain registration, I got mine from Google Domains. You have to register your home address to any domain you use.
Domains are also kind of a rabbit hole. But once you have one registered, you can go to your GitHub Pages and set the domain to the page. Mine costs $12/year and came with a free email from google.
And for launching your project, Memo 3 suggests either making it it's own JAR executable, or a windows Executable. I'm not sure how you would be able to put that on your website, but I'm confident there's a way to allow someone to download your product.