Since I had the same question as everyone else, it seems like it must be using just the transcript. When asking about one of those "8k HDR" showcase videos (with no speech), Bard responds with:<p>> I'm sorry, but I'm unable to access this YouTube content. This is possible for a number of reasons, but the most common are: the content isn't a valid YouTube link, potentially unsafe content, or the content does not have a captions file that I can read.