The first thing you need to understand is that there are several different ways to trigger execution logic in Camunda. You can make a Java API call, subscribe to external events, make restful calls, or asynchronously execute work. In  Camunda, a Job is executed whenever a Timer Event is encountered or when a task, or activity, is explicitly marked as Asynchronous After or Asynchronous Before. Those are checkboxes in the lower bottom part of the general sub-tab when you view your task's properties in the Camunda BPM modeler.