Universal Control is now available in the latest iPadOS and macOS Monterey betas. If you’re not familiar, below is an embedded video of Craig Federighi, from last June’s WWDC, showing how it works (via 9to5Mac). Definitely worth watching.
To make this work, all you need is the same thing required by the Handoff and Continuity features: Bluetooth and proximity. Just enable Bluetooth on your devices and make sure they are close enough for Handoff to work.
Can’t wait for Universal Control to hit the iPadOS and macOS Monterey public releases.