Although you get a huge collection of system fonts and Google fonts to use from, Astra gives you the freedom to use custom fonts too.
In order to use Custom Fonts with Astra, you need to install the Custom Fonts plugin available in the WordPress repository. The option Custom Fonts will be available under Appearance. Upload a font file here that you wish to use.
Here is a great tutorial made by Adam from WPCrafter that explains how this plugin works: