Apple is now selling their new accessories for the new iMac models separately. Before you would have to purchase a new iMac model to get access to the new keyboard.
The new Magic Keyboard, Magic Mouse, and Magic Trackpad we’re debut alongside the new M1 iMac back in April with a few updates. The major update was the integration of Touch ID into the keyboard. You can still buy a new Magic Keyboard without Touch ID for a cheaper price if you wanted to.
The new Magic Keyboard with Touch ID only works with the new M1 iMac models. Intel based iMacs won’t be able to work with the new keyboard.