Your viva, in a nutshell.
A unique exam, arising in response to a unique thesis, written by a unique candidate. There has never been a viva the same as yours before; there will never be another the same again.
But there are regulations: the rules that frame how vivas have to happen. There are academic practices: ideas from research culture about what makes a viva good. There are expectations: built up from all of the stories of past candidates, relatively probable situations, structure and outcomes for the viva.
Your viva will be unique, but not totally unknown, not totally unexpected. You can never have total certainty for it, but the more light you can shine on the probable circumstances the better you will feel for your once-in-forever experience.