Finding Your Serial Number On macOS and OSX.
Click the Apple Symbol at the top left of your screen.
Click About This Mac.
Your serial will be in the highlighted area.
Alternatively, on older versions of OSX you will see a window like this.
Finding your Serial on iOS & iPadOS.
Press the settings app.
Press general > about.
You will find your serial number here. If you press and hold on your serial number it will allow you to copy it.