In your thesis, whatever the topic, whatever the structure, you’re trying to persuade your audience of something.
In your viva you are adding to the work of your research and the words in your thesis to persuade your examiners. You want them to believe the truth: you did the work and it has the value they’ve understood when they read your thesis.
Whatever their questions, you’re working to persuade them that you have ability and knowledge and your thesis is good enough.
Between submitting your thesis and succeeding in the viva you have to persuade yourself that you have ability, knowledge and a good thesis. Depending on your journey, your background and your circumstances when getting ready, persuading yourself that you really are good enough could be the hardest task of your PhD.
But that’s what you have to do.
And hard as it might be, given your experience so far, it’s a challenge you will rise to. Given everything else you’ve done, you can do this too.