What font does macOS Mojave use?
The system font used in the current version of Apple's software platforms is called San Francisco. It is a sans-serif typeface which is developed by Apple. It was first released in 2014. It is a proprietary in nature.

You can learn more about the typeface by checking these links:


Fonts - Apple Developer
Typography - Visual Design - macOS - Human Interface Guidelines - Apple Developer
San Francisco (sans-serif typeface)


From the Wikipedia page:


  It was the original system typeface of watchOS and tvOS and has replaced Helvetica Neue and Lucida Grande as the system typeface of macOS and iOS since OS X El Capitan and iOS 9
  
  Apple restricts the usage of the typeface by others. It is licensed to registered third-party developers only for the design and development of applications for Apple's platforms