Select Xcode -> Debug -> View Debugging -> Take Screenshot of (device name)
Select Xcode -> Window -> Devices
Select your device -> Take Screenshot
A screenshot PNG file will be stored to the desktop.
Select Simulator -> File -> Save Screen Shot
or hold these two keys: [Command] + [S]
A screenshot PNG file will also be stored to the desktop.
iOS/tvOS Device without a Mac
If you want to make a screen capture of an iPhone/iPad without connecting to a Mac, hold the [Sleep/Wake (Power)] button and press and release the [Home] button.
An image of the device screen will be saved in Photos app.