A couple of options here:
CDN
In your index.astro file, you can add both Bootstrap latest CSS and JS in the header and body sections respectively like so
<!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/[email protected]/dist/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-Zenh87qX5JnK2Jl0vWa8Ck2rdkQ2Bzep5IDxbcnCeuOxjzrPF/et3URy9Bv1WTRi" crossorigin="anonymous">
</head>
<body>
<h1>Hello, world!</h1>
<script src="https://cdn.jsdelivr.net/npm/[email protected]/dist/js/bootstrap.bundle.min.js" integrity="sha384-OERcA2EqjJCMA+/3y+gxIOqMEjwtxJY7qPCqsdltbNJuaOe923+mo//f6V8Qbsw3" crossorigin="anonymous"></script>
</body>
</html>
npm
You can install Bootstrap via NPM like so
npm install bootstrap@latest
And require it in your index.astro
file within the frontmatter fences
You can check out this starter template for more ideas on usage
https://github.com/twbs/bootstrap-npm-starter