Did you know now use your iPad as a second screen with your Mac thanks to a feature called Sidecar. You have the choice of making the tablet a second screen to expand your work area or mirroring what’s happening on your Mac.
This can happen either wirelessly or you can plug your iPad directly into your Mac using the iPad’s charging cable. Plugging it in will keep you iPad charged during use. Make sure both devices are signed into the same Apple Account.
Then click the AirPlay icon on the menu bar at the top of your screen. Select your iPad from the drop-down menu.
Your iPad will now be an extension of your desktop. You can move windows over to it just like you can with any two-monitor setup.
Move your pointer over the full-screen button 
of a window on your Mac, then choose the option to move that window to your iPad.
To turn it into a mirror display, click the AirPlay icon again at the top and choose Mirror from the drop-down menu.
To turn off the second screen, just tap the Airplay icon again and click on Turn AirPlay Off in the menu.