There’s a general expectation that a candidate will pass their viva if they’ve submitted their thesis. The pass rate is so high that reaching that stage is a really good sign that success will follow in due course.
But the viva is not simply like ticking a box on some paperwork, nor is it a simple process in general. Perhaps compared to the scale of the rest of the PhD journey we could say it was “a formality” but only with reference to that great scale!
Expect to succeed – but also expect your examiners to be prepared, to do their jobs, to ask questions and expect you to respond. Do the work that’s needed, following a pattern of work and dedication that you have demonstrated over the course of your PhD and perhaps the viva will feel – with hindsight – like a formality.