What are the design principles of character models?

Ever since I saw Big Hero 6, I’ve been wondering about this.

[spoiler]At a certain point in the movie, they all end up in the river, well, actually at the bottom of it.
Baymax then states that his human companions’ body temperature is abnormally low (I’m assuming he has infrared vision and can see bodyheat).

When they all go to Fred’s mansion, oddly enough, they don’t follow the most important step for a hypothermia victim, which is to change into clothing not soaked with cold water and remain in their normal outfits for the entire scene.
I find it hard to believe a mansion that size doesn’t have addditional clothing stored somewhere.[/spoiler]

Which got me wondering, when Pixar creates a character model, is the clothing part of the model and therefore a new outfit requires creating a completely new model?
Or is it like real life and there is an underlying physical model inside the clothing?