How can we turn off the Windows OS?

Select Start and then select Power > Shut down. Move your mouse to the lower left-hand corner of the screen and right-click the Start button or press Windows logo key + X on your keyboard. Tap or click Shut down or sign out and choose Shut down.

How do I get rid of Choose operating system?

Type “MSCONFIG” to search for and open System Configuration. In the System Configuration window, go to Boot tab. You should then see a list of Windows that ever installed on different drives in your computer. Select those that you no longer use and click Delete, until only the “Current OS; Default OS” is left.