ensure that Apple Pencil works as tablet device when iPad is used as sidecar on macOS

https://help.apple.com/macos/catalina/whats-new/?lang=en#sidecar says

Sidecar
Turn iPad into a second display
Extend your workspace across two screens so you can do more at once. To get started, click the AirPlay status icon in the menu bar on your Mac, and choose your iPad.

You can drag a window from your Mac screen to your iPad to take advantage of the extra workspace.

Sketch with Apple Pencil
Use iPad as a tablet input device to draw with Apple Pencil in your professional creative apps for Mac. The handy sidebar makes it easy to reach frequently used keys like Command and Control.