How to create a Class that I can reuse in multiple modules?
Your use case is where Drupal 8 services come into play. When you'd like to create classes/methods that can be used across Drupal by other modules and your theme, create a custom module that provides this class as a service. You can then get an instance of this service class by either using the static service getter \Drupal::service() within *.module or *.theme files, or dependency injection in other classes/services.