ARDepthManager is a monobehaviour (Unity component) that exists for developer convenience as a way to set up the behavior for resolving AR depth, such as which occlusion techniques should be used and how performance intensive the depth logic should be.
On the other hand, ARDepthRenderer is used if a developer finds that the convenience of ARDepthManager does not suit their needs, in which they would manage direct calls to ARDepthRenderer as well as its instances within their logic to handle AR depth.