How to turn on the Apple Magic Keyboard backlight

Adding to the advice offered by stevejobsfan0123, you can add a control to the iPad Control Centre - this providing immediate access to the Keyboard backlight. You can find information about customising the Control Centre controls within the iPad User Guide:

Use and customize Control Center on iPad - Apple Support

The keyboard backlight automatically activates when the ambient light is low enough. You can control how bright the keyboard is in Settings > General > Keyboard > Hardware Keyboard.