Regarding your question, the physical model that is used in Pianoteq involves a great number of parameters that are themselves controlled by the "macro" parameters you have in the UI. Those parameters are adjusted in such a way that the model comes as close as possible to the original instrument that is modelled. This is a common situation in scientific computing/engineering where the parameters of a given model have to be identified in order to fit the data produced by the physical phenomenon that has to be simulated. The way the parameters themselves are adjusted is quite complex: some are entered directly (for example the lengths), some other are first computed automatically and then fine tuned by ear and other means.

But i was thinking more about, how do they create a model for a specific instrument, how do they measure all the parameters for it, compute them, etc.?