When a predefined multiphysics interface is added from the Model Wizard or Add Physics windows, it adds the constituent interfaces and the Multiphysics Couplings node, which automatically includes one or more multiphysics couplings.

If the constituent physics interfaces are added one at a time, then it adds an empty Multiphysics Couplings node. When you right-click this node, you can choose from the available multiphysics couplings.