They are most likely are just using cubes, nothing fancy, which every 3d engine will handle.
A 3d engine will never be simple, it is a complex piece of software, but an engine could be more accessible. But to just render a simple cube (first steps into 3d rendering), I would recomment to start with some simple OpenGL or DirectX (these are the two major 3d rendering APIs to utliize the videocard rendering power) demos/turorials.
Here is a good, simple, OpenGL based, C++ based start: Nehe Legacy Tutorials
Yes, that are legacy tutorials (because there are newer versions of opengl available), but it is still a good learning source to get in first contact with 3d rendering. Once you have learned enough, you can switch to a newer API.
ah, Ok, I understand more now knowing that it's nothing super fancy, just something basic. Thank you for your help.
On the other hand
Im going to start with OpenGL because I know SDL very well, so I've been told OpenGL is a good direction to head in.
but what does DirectX offer over OpenGL?