You can’t know exactly what will happen at your viva before you have it.
But you can know about the many vivas of your friends and colleagues. Use stories of vivas in the past to help get ready for yours in the future.
From these stories you can see that vivas range in length. They’re fairly structured conversations. You can expect challenging questions. You can also know who your examiners are and what they do. You can’t know what questions they will ask, but you can get a sense of what they might want.
Altogether you can have a good idea of what your viva will be like. You can build up a realistic set of expectations rather than worry about the unknown aspects.