What happened to Action Center on Windows 11?
Action Center is no longer a thing on Windows 11. Instead, the experience has been replaced with Notification Center and Quick Settings.
Notification Center is the place where you will find all your notifications and calendar. You can open the experience by clicking the "Date & Time" button in the system tray or using the Windows key + N keyboard shortcut.
Quick Settings is where you will find quick access to common settings, such as Wi-Fi, Bluetooth, Battery, Brightness, Volume, and more. Usually, the settings in this flyout will depend upon the supported features on the computer. You can open the Quick Settings with the Windows key + A keyboard shortcut or clicking the Network and Volume icon.
Source - Windows 11: The ultimate tips guide | Windows Central