Once you install and activate the recommended Kirki plugin, all of Airi's options will be available for you in the Customizer.
Among them, you will find the options for fonts. Simply navigate to Customize > Typography > Font Families and pick the font families you want for the headings and for the body and watch them change on your website:
Furthermore, from Customize > Typography > Font sizes you can configure the font sizes for various elements on your website. Please note: most font sizes in Elementor blocks will have to be configured in Elementor itself.