Installing the Application Server automatically creates a domain administration server (DAS). By default, the Java Message Service type for the DAS is LOCAL. So, starting DAS will also start its default MQ broker.
Creating a new domain will also create a new broker. By default, when you add a standalone server instance or a cluster to the domain, its Java Message Service will be configured as REMOTE and its default JMS host will be the broker started by DAS.
The figure below illustrates an example default deployment with an Application Server cluster containing three instances.