First of all, You have to enter in enter in Bootstrap site
URL : https://getbootstrap.com/
Include CDN to HTML
When you scroll down the site, You would be able to see this section. Copy it and Paste it to your html file like this
<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>Bootstrap demo</title>
<link href="https://cdn.jsdelivr.net/npm/bootstrap@5.2.0-beta1/dist/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-0evHe/X+R7YkIZDRvuzKMRqM+OrBnVFBL6DOitfPri4tjfHxaWutUpFmBp4vmVor" crossorigin="anonymous">
</head>
<body>
<h1>Hello, world!</h1>
<script src="https://cdn.jsdelivr.net/npm/bootstrap@5.2.0-beta1/dist/js/bootstrap.bundle.min.js" integrity="sha384-pprn3073KE6tl6bjs2QrFaJGz5/SUsLqktiwsUTF55Jfv3qYSDhgCecCxMW52nD2" crossorigin="anonymous"></script>
</body>
</html>
Source File Download
Download the source files and then add it to your html file using link and script tag.
Source File VS CDN
In case of Source File, After making your website, When you upload the website files to your server, users have to get all css, js files whenever users connect to your website. In case of CDN, users don't have to download the BootStrap files everytime. Once they connected to server, BootStrap files are cached in your browser through CDN Link so It will make your website a bit faster than downloading source files.