There are regulations for vivas created by each university. There are expectations for the viva, general patterns of experience, that can be observed in the stories that graduates tell. There are norms we can derive from more considered inspection of stories with a departmental community.
These three aspects give a lot of certainty about the viva experience. Every viva is unique, but none of them are a great unknown.
And yet: questions are asked that can’t be anticipated. Corrections are requested that went unseen before submission. Opinions are suggested that have never been considered. And circumstances sometimes change, from simple logistical room switches on the day to last-minute changes to video vivas due to illness.
Expect the unexpected. You can’t discard the rules and patterns of experience. Embrace them, learn from them, but also remember that you have to expect that you won’t know any question until it is asked.
That doesn’t mean you can’t be prepared. That doesn’t mean you won’t be ready.
Learn about the range of possibilities. Rehearse to build your comfort for being in the viva. Expect the unexpected.