The viva process could perhaps best be described as non-obvious.
It’s not obvious what the process is like because viva experiences haven’t traditionally been shared all that much. It’s not obvious what to expect because regulations only tell one aspect of what goes on. Your examiners’ questions won’t all be obvious; it’s not possible to second-guess what they might latch on to or want to dig into. And there’s no way to find out everything that will get rid of the sense of not knowing what to expect.
But if you want to get rid of uncertainty you don’t need to find out everything. Read the regulations to get the big picture, and ask a few friends for their experiences to fill in the details. Learn about general viva questions and formats, and find out about your examiners’ research and interests to explore where their particular questions might come from.
Explore a little and you’ll see that a lot of what was odd or unknown about the viva is obvious with hindsight.