How to create an application that has some functionality and share it so someone else can put it on their website?

If I create a small web application on my desktop (html and js files) and want to share the functionality of it so my friend could use it on his website, how can I do that?

For example, if I want to use the benefits of bootstrap to quickly make changes, they instruct me to download this file

<!-- JavaScript Bundle with Popper --> <script src="https://cdn.jsdelivr.net/npm/[email protected]/dist/js/bootstrap.bundle.min.js" integrity="sha384-ka7Sk0Gln4gmtz2MlQnikT1wXgYsOg+OMhuP+IlRH9sENBO0LRn5q+8nbTov4+1p" crossorigin="anonymous"></script>

1. What is this file? I’m assuming all the js code and logic to their
   platform lives in this file, yes?

2. If I want to build my own
   application and share it so others can use it, how would I do it?

Sample application: a user copies the CDN file link, they add it to their index.html file at the top of the page, when they reload their website a modal pops up in 5 seconds. That’s it.