Knowing exactly what to say to answer every question in your viva isn’t a reasonable expectation. It’s not required for the viva. Your examiners don’t expect it from you. You would probably need to know all of the questions before they were asked (and you won’t).
You’re not expected to know all the answers, but you are expected to respond to every question.
A response could be an answer or an opinion. A response could be sharing an idea or offering a hypothesis. A response could be a gut feeling or a question for clarification.
A response could even be saying “I don’t know,” and then explaining why.
You can’t have all the answers but you have many options for offering a response.