Registered developers can download the beta through the Apple Developer Center and after the appropriate profile is installed, betas will be available through the Software Update mechanism in System Preferences. There is also a macOS 11.6.5 update for those still running macOS Big Sur.
macOS Monterey 12.3 is a major update that introduces Universal Control, a feature that Apple first teased at WWDC and then delayed for several months. Universal Control is designed to allow you to use a single mouse/trackpad and keyboard across multiple Macs and iPads (with iPads running iPadOS 15.4).
After updating your devices to macOS 12.3 and iPadOS 15.4, Universal Control works automatically, allowing your Mac’s cursor and keyboard to work on the iPad and vice versa. It’s simple and intuitive to use.
maOS Monterey 12.3 adds support for updating the AirPods firmware when the earbuds are connected to a Mac, plus it deprecates kernel extensions used by Dropbox and Microsoft OneDrive, and it eliminates Python 2, with Apple instructing developers to use an alternative scripting language like Python 3 going forward.
There are also new emoji characters, including melting face, biting lip, heart hands, bubbles, beans, face with diagonal mouth, palm up hand, eggs, troll, low battery, coral, lotus, and more.