is zbrush strictly characters or can you do environments?
Zbrush is a sculpting software, it's very common to use it for creating static meshes used to decorate environments. It isn't suited for assembling and managing meshes into vast environments like those you see in contemporary games and movies though.
even if you produce a displacement map and then decimate the mesh? or did you mean something else?
Meant stuff like city blocks populated with buildings with interiors and details. Outdoor scenes with winding roads, trees, foliage. Stuff like that.
You may well use Zbrush to create elemets for such environments but to assemble and render them you need a general 3D software.