This file's HTML is copied and adapted from the Bootstrap 4 introduction. It uses local versions for all the Bootstrap files. There are no CDNs.
When you use a CDN for a linked resource (a file, such as a CSS file), you use a complete URL — an absolute path — to load the resource.
More info: What is a CDN?
You don't need to use a CDN. You can download the Bootstrap files and put them in your project's /css/ and /js/ folders.
Note that you always need the Bootstrap CSS file, but the JavaScript files are often NOT needed. It's best NOT to include the JavaScript files when the page doesn't need them.
Note that Bootstrap top margins are always 0. You can add padding or margin with the Bootstrap spacing classes.
Resize this page to view the Bootstrap container's behavior.
This file's HTML is copied and adapted from the Bootstrap 4 introduction.