A process instance is started (based upon a process definition) through e.g. the startProcessInstanceByKey method on the runtimeService.
The runtimeService can be retrieved from the ProcessEngine class, which gets built by a ProcessEngineConfiguration. The ProcessEngines class is just a simple helper class to build such an engine (but you can do this programmatically yourself).
The engine will inspect the definition and when it reaches a certain step (let’s say a user task), it will execute the associated behavior (e.g. the UserTaskActivityBehavior).