If you see the same thing on both displays, you should start by making sure the displays are Extended instead of Mirroring ...

With macOS Catalina's Sidecar feature, your iPad can be a second display for your Mac. You'll need a relatively recent Mac, but here's how to set it up and use Sidecar.

Duet Display lets you use any Android device as a second screen for your Mac, whether wired or wireless, giving you extra space for multitasking.

Learn how to stop an iPhone screen fading to black at inopportune moments by extending the screen lockout time.