Apps and widgets are generally opened via views which act as a selection menu. More precisely you open individual modifications (mods) depending on the settings either on full screen or in a window.
Navigate in SystemView to get to Mod Overview. Change filter settings if required to view a specific mod. A tap on the start button opens the chosen mod of a prevailing app. Mods can be started directly in AppManager as well as their prevailing views.
Apps resp. mods starting on full screen can be closed in several ways. Either press the respective key on the keyboard (see list of keys) or use the terminal.