UI is short for, "user interface."
PS Touch projects are self-contained within the app itself. If you want to add one element from one project into another, you'll have to export the project to Camera Roll (Gallery if on Android) beforehand. Even then, they'll be flattened since standard image file types like JPEG do not support layers let alone transparency.
Because of this, bring all the raw elements together in one project first before doing any editing. (e.g., Don't use the Scribble Selection Tool until you've got all the layers/images together.) You can import/insert up to 12 layers.