Product Backlog refinement is a continuous activity. When an item is at the top of the backlog, which means it will be developed in the near future, more refinements are usually needed to make sure it is clear and contains just enough
details (not all possible details). It's preferred to have the items that will be selected in the Sprint Backlog completely refined before Sprint Planning; however, it's OK to have more refinement done during the development. Remember
that there's no such thing as Sprint 0 and there's no time between two Sprints that can be spent on something such as refinement. The team is also cross-functional and does the a-z of the work needed for each item, including refinement;
they don't use people from other parts of the company.