Do you want to take screenshots on a Mac, but without shadow around the window? Then you can disable this by using a terminal command. Since macOS Mojave you have to use a different command than under macOS High Sierra (more info).
Sidecar lets you extend your workspace by using your iPad as a second Mac display. All the ins and outs about SideCar are explained in this Apple document. Unfortunately not all Macs are compatible for this feature, but there is a workaround that might enable Sidecar. With the use of a terminal command you can use all Macs that can install macOS Catalina for side car. But be aware, it doesn’t always work, but it is worth trying.