Vivas aren’t a great big unknown. There are patterns of experience: for example, they tend to be two to three hours in duration, often begin with similar opening questions and typically result in minor corrections.
Yours will be unique though. It will probably fall within a range of expectations. It won’t be totally unknown or unanticipated, but you won’t know what will differ or how it will differ until you experience it.
Your viva will be unique, not unknown. You have to balance what you learn about viva experiences with the knowledge that yours won’t be quite the same. You can have reasonable expectations, but no guarantees of what yours will be like exactly.
Listen to stories, read the regulations and build up an idea of what a viva is like. Prepare for the general event. You don’t have to be prepared to hit a single target: you can be ready for whatever presents itself when you meet your examiners.