Peel an onion layer back carefully and there’s more underneath. Again and again, not infinitely but quite a way. It takes patience and effort to go down layer by layer.
The same’s true with your research. One can see the end result, but with patience and effort you can dig deep into what’s there. It’s built on a lot. You can ask why many times. So can your examiners. They can dig deep and explore motivations, assumptions, the fundamentals. It’s not for fun, it’s for their understanding and to show yours.
You can be ready for the viva by digging below the surface, prepare by examining the background. Test your assumptions. Remember why your work is valuable. You don’t need to go too deep though, going further and further, ad infinitum. That way lies a lot of tears.
Which brings us back to onions.