Is there a keybind on Steam to open Big Picture?

Yes, there is a keybind to open Big Picture Mode on Steam. By default, you can press the "Guide" button on an Xbox controller, or the "Steam" button on a Steam controller. If you're using a keyboard, you can also access Big Picture Mode by navigating to the Steam menu and selecting it, but there isn't a default keyboard shortcut. You can customize keybindings in the Steam settings if desired.