I want the film I edit in the VSE to be the background of a scene to move objects inside a real moving landscape or to make a title. What settings ? Thanks.
To preview your clip in the 3D viewport, set it as a background image/movie.
To render your scene, add the scene as a layer in the VSE above your movie clip. Set the scenes blend mode to alpha over.
Alternatively to the VSE you could combine your clip and 3D scene in the compositor. It is the same principle.