Apple has released macOS Monterey, versioned as macOS 12.0.1, to the general public. The build number is 21A559.
Any Mac that is compatible with macOS Monterey can download and install the update right now if they choose to. Users can also stay on their current macOS version if they prefer by simply ignoring the update. If interested, you can learn a bit more about preparing a Mac for MacOS Monterey.