https://developer.www.ads-software.com/reference/functions/wp_enqueue_style/
In particular, review the More Information > Notes section here:
https://developer.www.ads-software.com/reference/functions/wp_enqueue_script/
There are useful examples in the user notes sections near the end of each page.
The code can go in functions.php of your theme, or in a custom plugin. Instead of altering your theme, it’s better to create a child theme. TBH, if you will not be modifying any templates, a custom plugin is easier to create.
To create a child or plugin, you need FTP or your hosting file manager because new files need to be uploaded. You can edit your existing theme with the theme editor. It’s recommended you switch to a different theme before changing anything in your normal theme. If there are any errors in the changes, the theme will not reactivate. Errors made in an active theme could lock you out of WP, requiring FTP to go in and fix the errant code.