If you have your own theme, then you'd add the <link> tag that Google Fonts gives you to one the head functions in qa-theme.php (e.g. head_links). Then add the CSS part in your CSS file.
If you're using a pre-built theme, perhaps the simplest way would be to add the code in the box in settings, at "Admin > Layout > Custom HTML in <head> section of every page"
For example using the Roboto font:
<link href="https://fonts.googleapis.com/css?family=Roboto&display=swap" rel="stylesheet">
<style>
body { font-family: 'Roboto', sans-serif; }
</style>
