You had the right right idea, just type at symbol and the username, pick it from the list that comes up.
No, Big Box does not play/draw the emulator in it's screen. It opens the emulator. Most people use Retroarch because it easier to keep up to date and does 10ft interface well. Also defaults to most of the settings you would want anyway. If not using retroarch I'd pick an emulator that can either be told to launch fullscreen (exclusive if you can) via the command line, or in the settings.