Here I want to show with a few examples what is 3D and what is 2D elements.
First off we start making the model in 3D and populating it with objects.
Then we apply materials on the different models. Below is the same model as above but with a different material, without lights and environment.
When this is done we apply lights and reflections that comes from an HDRI Dome image (that we also create when needed), to make it photo-realistic. And then we render the model in 3D. And the post-process of painting the rendered model together with the backplate (background image, wich we walso made for this) starts. And we end up with something like this.
There are more ways we use when creating images, but this one is a good example of a basic workflow.