a window can be made to exist outside the game, for example, you can keep it in a second monitor. it can also be made embedded, it will then be restricted to the game window. it supports theming.
a popup is a window, but it’s designed for use with things like tooltips and menus, in fact those inherit from popup.
no. do not ever show/hide a Control unless it for something specific like during drag_n_drop or to resize the UI. use something like a tab for managing different views. for moving menues around like a toolbar, delete or add the toolbar.
for a window you don’t need this, but for toolbars use containers.
for this case use a popup or window.