They read your thesis. Think about it. Make notes. Think some more.
Think about topics that need to come up in the viva. Check some of your references, and maybe do some more background reading. Think about specific questions to ask in the viva.
They think about whether or not your thesis is good enough. They think about what to do on the day of the viva. They write a report and think about what the likely outcome of the viva.
Your examiners understand what this all means in terms of your thesis, the process and the outcome of the viva. They understand what your viva means to you.
Your viva is a big deal for you. It’s not trivial for them either.
They take your viva seriously, and although it could feel overwhelming that they do so much, it really is good for you.
Your examiners do their job.