Actually, this model is made without any background reference images but enough knowledge of human figure makes this possible. Modeling the human body or any model in that case requires knowledge of it beforehand. Even with the background image references, there are still some details (like the armpit) that is not easily visible or are difficult to comprehend with just simple orthogonal projections. It is advised to make some study on basic human figure and proportions first.
For those who likes to have a background reference image to use while modeling, use this orthogonal views of the model. You could still try and test your knowledge on the human figure later by trying to model one without image references.