I’d like to record a video of a real person in a virtual 3D environment similar to the picture attached. I know that the Unity marketplace sells assets for the Moon and Mars. How can I put a real person in this scene?
You will have to make the video of the real person into a sprite and put the sprite on a quad.
You might also be able to test projecting the video of the real person with a projector, although this is probably not your desired look. Some tests will be required for both options mentioned above.
Same as if you were putting a real person in a 3D environment in a 3D application.
You will have to solve the shadow issue with sprites casting onto 3D object - but that is another subject.