So the problem is that you already created a website using html and have problem of how to make your site mobile friendly so that people can open your website in their android looking like this.
All you have to do is just add this line of code in your index.html
<meta name="viewport" content="width=device-width, initial-scale=1">
Before the code implementation, you had to zoom in to see the content but now the content already zoomed in by default if you view it in your mobile devices. And this is the beauty of using bootstrap.