Every viva is unique.
Some vivas are long, some are short, but most fall within a certain range. Some vivas start with a presentation, some with the candidate being told they’ve passed, but most begin with a simple question to start the discussions. Some candidates are excited, some are unprepared, but most are nervous-but-ready for the hours ahead. Some candidates get no corrections, some resubmit their thesis, but the vast majority are simply asked to complete minor corrections.
A variety of viva experiences leads to a range of expectations. There are lots of things that could happen: thankfully there are patterns of experience that stand out. Expect to be in your viva for several hours and expect questions that prompt discussion. Expect that you will be ready-but-nervous (or nervous-but-ready!) and expect that you will have to make some changes to your thesis afterwards.
Your viva will be unique but cannot be a total unknown. Understanding the range of expectations for the viva process can help you to prepare for whatever happens.