https://www.macrumors.com/guide/universal-control-vs-sidecar/
..With Universal Control, you can use the same cursor and keyboard on any nearby Mac or iPad that’s signed into your iCloud account, and it works between multiple Macs and between multiple Macs and iPads positioned within 10 meters (30 feet) of each other…
Sidecar works quite differently. Announced in 2019 as part of macOS Catalina, Sidecar is designed to let you use an iPad as a secondary display for your Mac. Up to one iPad can be used to either mirror the content on your Mac’s display or turn it into a secondary display for extra screen real estate in macOS. Universal Control does not extend your Mac’s display to the iPad, so if you want your Mac’s content displayed on the iPad, you’ll need to use Sidecar…