The ALT-TAB feature in Windows is primarily used for switching between open applications rather than minimizing them. When you press ALT-TAB, it allows you to cycle through the currently running applications, bringing the selected one to the foreground. However, this does not minimize the windows; it simply changes the active window.
If you want to minimize a window, you can do so by clicking the minimize button (usually represented by a dash or underscore) on the window's title bar. Alternatively, you can use the keyboard shortcut Windows Key + Down Arrow when the window is active to minimize it. If the window is maximized, you can first restore it using Windows Key + Down Arrow and then press it again to minimize it.