Since none chimed in (in the past 6 minutes) , I, an idiot, will share what I think I know. When reconstructing the faces of people from a skull, either with clay or software, they model the various tissues–muscles, fat, skin, etc according to models based on samples. How they would do this for a creature that isn’t very like any current living creature I don’t know. It is probably educated guesswork?
I just read an article on this process for a neanderthal and in that particular instance they used data from humans since I guess it was close enough.
But, for example (referencing a recent meme) how do they know spinosaur had a sail and not a hump back and neck muscles like a buffalo?? Seriously though I’m sure they can tell which bones have attachment points, how much force they can withstand, etc.