That’s the viva in a nutshell.
You invested years of work into research and months into writing up. Now, for a few hours: say more, do more. Say more about your work; do more in thinking and understanding and demonstrating that you’re a capable researcher.
Not for long, not on the same timescale as before. It might feel pressured but remember that the years of work help you to say more and do more for a few hours more.