Think about what a viva is for. (btw I’ve been an examiner so I know what I’m talking about).
It’s not a memory test. The first purpose is to establish that this is your work and that you understand what you’ve presented in your thesis. Any half decent examiner will use all the usual questions about ‘what is your main argument?’ ‘Why did you choose this topic?’ to establish that. These aren’t meant to be trick questions. You can tell them you are nervous - they should understand. If you realise you’ve got off track or not given your best answer you can say ‘can I answer that question again? I want to make sure ‘x’ is clear.’
The second purpose is to establish the ‘original contribution to knowledge’ that you need to make at PhD level. Examiners will establish this by asking about other work in the field and again about your argument. Why did you engage with this scholar in this way? What did you think of x’s work? They might directly ask what is your contribution.
They have largely already made up their mind about this from your thesis. That could seem scary but it could also be freeing. You don’t have to convince them from scratch - your thesis has done 80% of the work for you, probably more. They want to clarify some points and check why you made certain choices.
The third purpose is to determine exactly what corrections need to be made. Major corrections is the most common outcome in many fields - it’s a perfectly normal outcome. Minor corrections is also common. Again, the vast majority of this is decided in the examiners’ reading and reports before the viva. The viva is a positive opportunity to address some points so that these don’t get added to the list of corrections.
So, I might ask someone in the viva ‘I notice you only mention ‘b’s’ work quite briefly in chapter 3, why was that?’ If they say ‘I don’t think b is relevant’ but can’t give a reason why, then there might be a correction there to check b’s work and address it through an additional paragraph or a footnote. If they say ‘I looked closely at b and decided they didn’t need to be more than briefly mentioned for this reason’, I’d probably strike that off as a correction altogether.
Some areas that could be subject to corrections could be tackled in the viva like this, others are going to be addressed afterwards in writing whatever you say or do.
You can’t guess everything they are going to ask you about and you don’t need to. Just give the best answer you can. You can say you don’t know or that you need a minute to think. Obviously you ideally won’t do that for every question but if you get an unexpected one that’s fine.
Overall, any half decent examiner sees the viva as an opportunity for you to do well and to show what you care about. They don’t want a horrible viva experience any more than you do. Sometimes examiners are not half decent. Sometimes they are shit and treat you badly. But your control over this is limited and it is the exception rather than the rule. Even then your internal or independent chair is there to add another perspective.
In terms of how to prepare - practice, practice, practice, out loud, with other people. I know this is hard to arrange but bribe friends and family! Mark up your thesis with coloured post it’s and refer to it as you answer. You can also write notes in the margins/use highlighter and refer to them. Again, it’s not a memory test!
Good luck!