Expect your examiners to be professional.
In the same way that a candidate can be expected to get ready for the viva, expect that your examiners will do what they need to be ready to examine you.
If they’re a relatively new academic then they will receive training to be an examiner. They’ll rely on their colleagues to help them explore the role. If they’re a little disconnected from your area or topic of research, they will do their homework in advance of the viva.
An examiner might not know everything about your field before they read your thesis, but they will learn enough to be a good examiner for you. If they’re busy they will make the time. If they’re uncertain, they will dig deeper.
If they’ve said yes to being your examiner then you can expect they will be working to do the job well.