In ADS, what are the differences between the lumped components and the lumped component models?
A model is not a circuit element. It doesn't have any terminals that can connect it to wires. It just collects some parameters so that actual circuit elements can be made alike by referring to the model.

It's not common to use models for passive elements, but it's very common to use them for active devices like BJTs, MOSFETs, or diodes.