You may have a MacBook with a small screen or a Mac mini connected to a TV or external monitor and you want to do this. Use a second screen or monitor second device to expand or mirror the current display. With macOS Catalina and an iPad, this is possible thanks to the new "Sidecar" function.
Sidecar extends your display from your Mac to your iPad, giving you more room to work or play. Alternatively, you can mirror or duplicate your Mac's display on your iPad so you do not have to sit in front of your Mac to continue working.
With the Mac screen expanded or mirrored, you can: familiar touch gestures on your iPad; Edit text with the new text editing features in iPadOS. Use an Apple Pencil on your iPad to work with Mac apps on the screen. A sidebar displays Mac modifier keys, such as Command, Control, Shift, and Option, on your iPad to help you control your Mac. A touch bar provides app-specific controls at the bottom of the iPad screen, even if your Mac does not have a touch bar.