You can’t know in advance what your examiners will ask for every question. You can’t know how long it will be exactly. You won’t know what parts of your thesis they think need changing, amending or updating. You can’t know how you will feel when you’re there.
But you know what it’s all about. Your thesis is a guide to what might come up. A guide doesn’t tell a person everything, but it lays out what is important. You can know many of the topics that will come up in your viva because you wrote your thesis. You can know what happens in vivas because there are regulations for your institution and expectations in the wider academic community for what vivas are supposed to be like.
You can’t know everything about your viva in advance. That doesn’t mean your viva is an unknown.