I will answer in the context of my PhD defense and the defenses of my students, who are in various versions of public health.
There is a separate session with the graduate committee. This can last a long time, or it can be fairly brief, and in my experience, there's not a strong correlation with length of time and success.
In that session, somewhat harder questions, ambiguous questions, etc. can be asked. What would you do in X circumstance. Push the conclusions of your study a little farther. On occasion, and I dislike it when this happens, there's objections from one or more faculty members about core issues in the thesis. It's far less structured than a presentation, and is, while not hostile, definitely critical. I got asked several questions with the intent to stump me, including one that was intended as a lesson for the future.