It really depends on where you’ve set the services to execute. Most of the time, several services are set to execute in the same location like the receive handler or send handler of a port, in which case the esb dispatcher pipeline component will execute all the services one after the other according to the itinerary scheduled. In those cases, routing isn’t really an issue.
When routing is needed because you’re switching locations (say, you’ve got one service set in the receive handler of the receive port, then move to an off-ramp or to a service in an orchestration), then standard BizTalk Pub/Sub handles it based on the message properties and the subscriptions that ESB on/off-ramps and special orchestrations have in place.