If a diaphragm is modeled using plates, will it simulate a flexible or a rigid diaphragm ?

When one defines a plate, the actual thickness of the diaphragm is modeled through plate thickness definition. So the true stiffness of the slab is considered in the analysis and diaphragm shears are distributed accordingly. If the thickness is small, the diaphragm would behave more like a flexible diaphragm and similarly as thickness is increased, the slab would tend to behave like a rigid diaphragm.

The software does not idealize the diaphragm as rigid or flexible. However you can always compare the maximum lateral deflection of the diaphragm to the average lateral drift of the adjacent vertical elements and thereby classify it, if you need to.