Without having used Zoom much I can't definitively say how it works, but I've used BlueJeans quite a bit and noticed compression artifacts in various parts of the UI (e.g. text underneath each video source). That means BlueJeans is definitely muxing video sources and it really does not have a noticeable delay when changing the view. Since each video is already so compressed I think they can get away with sending you really low bitrate streams during the transition and you'll barely notice.