Willpower was related to consciousness and mental strength. The higher one's cultivation level was, the stronger their consciousness and willpower would be. For example, a Body Integration cultivator could easily resist illusions cast at the Void Refinement Realm, but consciousness and mind could not determine the strength of their willpower. Even ordinary people could have extremely strong willpower.
However, most people's willpower was relatively ordinary. They would not be able to withstand powerful pressure. Only a small number of people had firm willpower. Even if they were humiliated in all ways, they would not submit.