B-roll can be shot before. I usually describe it as the video in shows and moves that does not involve named talent. Once participated in a food show, B-roll was shot six weeks before, the whole show without the star was shot, and the proprietor actually cooked his specialties then. When the star came in the proprietor just threw onions in butter and gave the star a taste of some already made food.In the filmmaking industry, "B-Roll" are takes and scenes shot in addition to the principal action scenes and takes. It's done for "establishing" (gives visual location context), supplemental, and alternative camera points of view to intercut with the principal takes. In the movie industry, this is sometimes done after main production in "pick-up" shots requested during editing to facilitate good transitions between main scenes.
The primary or main scenes and takes used to be referred to as the "A-Roll" but that's rarely used now (the "Roll" is a reference to film roll). B-Roll is often done in motion picture production by a "Second Unit" with its own director, and sometimes shot with no sound which would be added later in post-production if its used. For stuff like YouTube and news videos, B-Roll is shot after the main stuff is recorded, to be intercut with it for visually supporting the person doing the talking or interview or reporting. Sometimes the main audio is overlaid onto B-Roll video. In news footage made for broadcast later, when the reporter continues narrating as some other scene is brought up to depict what the reporter is talking about, that visual is often B-Roll that was shot separately. When there's a disconnect between the narrator and scene being depicted in a prepared news segment, it's typically an error in editing made by the editor grabbing the wrong B-Roll video for it.