You do the research and you write a thesis.
Two examiners, one from your institution and one from elsewhere, prepare to examine you. They read, make notes, think and discuss what they need to to do at the viva.
On the day, they need to lead a discussion. They need to ask questions that explore, clarify and assess.
To be ready, you need to prepare. You need to read, think and rehearse. You also need to remember that you did the research and you wrote a thesis. At the viva you could be nervous but you can feel confident because of the thousands of hours you’ve already invested to get this far.
There’s a lot to say about the viva, of course, but the basics cover a lot.