Patchwork
Every viva is different because every thesis and candidate are unique. Your thesis and experiences will to some extent ensure that your viva is different from every other viva before or after yours.
Every viva follows patterns because of university regulations, general expectations and departmental norms. There’s a patchwork of rules and ideas for what a viva is supposed to be like that gives every viva some structure. Taken together, each of these elements tells a candidate roughly what to expect: how long it might be, what kind of questions could come up and what the experience might feel like.
The more you stick pieces together, the better informed you can be and the more ready you can make yourself – while understanding that you won’t really know what will happen until you get there. The patchwork of regulations, expectations and norms helps you be ready for whatever happens.