Wiring up an external font for your Drupal 8 site is a lot more straightforward thanks to libraries. In my case, I wanted to add two Google Fonts in addition to my theme styling. Therefore, I'll need three libraries: core, librefranklin, and opensans.
Next, define the global libraries in the themename.info.yml file in the root of the theme. Because these libraries are added in the themename.info.yml file, they will be included on every page. Check out the link the below to see how to add libraries to specific pages or sets of pages.