At this level, the design focuses on the implementation of classes, mainly fields, and methods. It is full of details with concrete and low-level information. The source code is the main representation of the design at this level. You might create some UML diagrams for complicated logic, but you should keep them minimal because details change, and they change a lot. You certainly don't want to put too much effort into having to update those diagrams. Instead, you should invest more time in the source code itself. Keep it clean, simple, and expressive.

And at this level, because of these changes, there will be a lot of refactoring going on to keep the design as good as it can be.