Fonts

With the default editor you don’t. Themes manage what font is used for various kinds of content through CSS rules. What you can do is add a custom CSS class to certain blocks, then apply appropriate CSS rules to use a specific font for only that class. Add any such CSS to the Additional CSS section of the customizer.

Kind of a pain to come up with the CSS, but once you do, you can reuse the same custom class to apply the same rules elsewhere.