To open a recently used file in libreoffice they are shown in the main part of the window. To open a program or to create a file from a file dialog are along the left hand column.
Libreoffice all of the different components have a window menu that lets you switch between a document in libreoffice writer and libreoffice calc. To do this go to :menuselection:`Window` and sleect the window that you want to switch to.