The Windows key opens the Start menu and the taskbar just happens to appear with it.
If a certain element within an app can go full screen e.g., a video player on website in your browser, the shortcut(s) will show the taskbar in that case as well. This will work for practically any and all apps, even games. The Win + T keyboard shortcut will focus the apps on the taskbar and the Win + B keyboard shortcut will focus the items on the system tray or if they’re collapsed inside the menu, it will focus the little arrow that reveals them. Like the Windows key on your keyboard, they do something else. These two shortcuts aren’t meant to actually show the taskbar. To dismiss it, you have to click inside the app that is full screen. This will show the taskbar but it won’t automatically dismiss itself. The two keyboard shortcuts that you can use to show the taskbar in full screen are Win + T and/or Win + B. If you want to show the taskbar in full screen, but don’t want the Start menu to open, you can try two other shortcut keys. Tapping the key again will hide them both. If you tap the Windows key on your keyboard though, the Start menu will open, and t he taskbar will also appear. That is the whole point of the full screen you get an uninterrupted interface to work in, or watch videos. In full screen, no other UI elements of the OS are visible.
