If you order a sandwich in a cafe, you know there’s a certain set of expectations. Bread of some sort, in some shape; often two pieces or parts with a filling between them, generally simple and quick to produce. Lots of variety, but through experience you know the kind of thing you’ll receive.
The same is true of vivas, even if you’ve never had one before. You can ask others about theirs to get a sense of what to expect. Read the regulations for your institution, especially if your viva will be over video. Listen to stories to find out the common experiences for your department.
Questions vary, but vivas follow patterns that aren’t too hard to learn about. Every viva has qualities you would nearly always expect, like an internal examiner and an external examiner. Together they facilitate a discussion – with you and your work sandwiched right in the middle.