Universal Control will not be accessible until spring 2022, according to Apple's website. The delay might come as a disappointment to those who were hoping for more seamless connectivity between their devices in the near future. Still, rather than publishing a possibly defective version, it's best to make sure the feature works properly.
The feature will be available on MacBook and Macbook Pro (2016 and later), the 2019 Mac Pro, MacBook Air (2018 and later), iMac (2017 and later) and the 5K Retina 27-inch iMac from late 2015. As for supported tablets, you'll be able to use iPad Pro, iPad Air (3rd generation and later), iPad (6th generation and later) and iPad mini (5th generation and later) with Universal Control.
All of your devices must be logged into iCloud with the same Apple ID. If the devices are within 30 feet of each other, you can connect them by USB or use Universal Control wirelessly.