Every viva is a custom exam to examine one particular person and their thesis. But every viva takes place according to practices that are consistent across the UK, and according to regulations and expectations that are consistent with an institution.
Different and the same.
Taken together this creates a slightly head-scratching puzzle, but not an impossible one. To solve it for yourself, first check your university’s regulations to see what to expect broadly. Then talk to friends to get a sense of what their vivas were like.
Finally, realise that your own will be unique. The expectations create an environment for you to thrive in. The variety comes from you and your work, not from a lack of rigour.