They’re not always looking for answers.
Viva questions aren’t unfair, but they might not always be asked with the expectation of a definitive answer. It could be they simply start a discussion.
You might not know, your examiners might not know, maybe no-one knows “the answer”.
But you might have an idea or two. Or you might know why you don’t. Or you could discuss the topic with your examiners.
Another secret: you’re among the best people – possibly the best person in the world – to discuss the questions that are asked in your viva.