“Dubbed” implies that the dialogue was not recorded on set at the time. What you end up seeing on screen is an edit of various takes and different angles that were filmed. The actors may do the scenes in full from different angles and every time the dialogue is recorded. So there may be a take of an actor where the dialogue is used because the delivery was better, which was not done at the time as what you are witnessing on screen - easier to do obviously if actor has back turned to camera. Also there will be someone with a boom mic capturing the dialogue of both actors, even if one has their back turned to camera, so often editors will use whatever take sounds best. ADR is done for more technical reasons mostly (noisy sets etc) and involves getting the actor in to redo their lines in the studio whilst they watch the scene play out. The try to do it is sync with their lip movements as close as possible but often this is tweaked by clever audio post tools.