Show Them What You Know
At your viva you’re expected to explore your significant original contribution with your examiners, tell them about your PhD journey and demonstrate for them that you are a capable researcher.
Which is simple to understand but sometimes difficult to do!
You have to show your examiners what you know. Show them what you understand. Show them what it means.
Whatever their exact questions are you know the topics they will be interested in. Part of the challenge is being ready and able to respond well, whatever the question might be.
By the viva, you’ve done the work. You’ve done the prep. Take a deep breath and show them what you know.