Content delivery network (CDN)
You can learn here, what's a Content delivery network (CDN). They will make your website faster. As a programmer, you can even use one for free, as I'm doing with this website.
CDNs are (usually) faster than hosting locally the files. You can also set up your local server to reduce the GAP with CDN (by using good headers for images etc.). You got a lot of notes about this on the WEB.
CDN for libraries
For instance, for Bootstrap, you can download the css/js (as you may have been doing only now), but if you're reading further, you can use a link
<link href="https://firstname.lastname@example.org/dist/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-KyZXEAg3QhqLMpG8r+8fhAXLRk2vvoC2f3B09zVXn8CA5QIVfZOJ3BCsw2P0p/We" crossorigin="anonymous"> <script src="https://email@example.com/dist/js/bootstrap.bundle.min.js" integrity="sha384-U1DAWAznBHeqEIlVSCgzq+c9gqGAJn5c/t99JyeKa9xxaYpSvHU5awsuZVVFIhvj" crossorigin="anonymous"></script>
That's it. It is usually quite easy.
CDN for your assets
Note: assets are your stylesheets, your images, your videos, your audios, etc.