How to switch to underlying window of the same application?
You can switch between an app's windows using a keyboard shortcut, a menu bar item, the app's Dock icon, and Mission Control.

The keyboard shortcut to switch to the next window behind the current one is cmd+`. That corresponds to the Window-> Cycle Through Windows menu bar item. The menu item isn't available in all apps, but the shortcut usually is.

You can also use the Window menu in the menu bar to go directly to the window of your choice. Each window is shown there, and you can click on its item to switch to the window.



A similar menu is available in an app's Dock icon's contextual menu (access by clicking and holding, or right/control-clicking):



You can also acces visual previews of all of an app's windows and click on the preview of the window you want to switch to. To do this, press F10. This brings up Mission Control's Application Windows function. You can also access this function by the Show All Windows item in an app's Dock icon's contextual menu.