How is occlusion culling handled by this engine? If I use a fully 3D mesh as a wall for example than will the sides of the mesh not seen inside the level still be rendered? Can I use full 3D meshes as walls/floors/ceilings etc or should I use flat/1 sided meshes for that?
Occlusion culling is done via hardware occlusion queries. Bounds of the mesh are tested against depth buffer.
Models with least number of triangles are surely preferred, but quite often watertight meshes are used in favor of having less issues with light leaking.
How is it done with the forward renderer? For example on mobile or some VR?
We have several methods of visibility and occlusion culling in the engine. Please see our documentation for detailed explanations of the methods we use: https://docs.unrealengine.com/en-US/Engine/Rendering/VisibilityCulling/index.html