There's no central stage controls on iPad Air 4th gen?

Your fourth generation iPad Air does not support iPadOS Stage Manager. Stage Manager is available for these models of iPad:

• iPad Air (5th generation)
• iPad Air 11-inch (M2)
• iPad Air 13-inch (M2)
• iPad Pro 11-inch (all generations)
• iPad Pro 11-inch (M4)
• iPad Pro 12.9-inch (3rd generation and later)
• iPad Pro 13-inch (M4)

More information about Stage Manager and supported models can be found within the iPad User Guide:

Move, resize, and organize windows with Stage Manager on iPad - Apple Support

Centre Stage settings are accessed from the Control Centre whilst a supported App (such as FaceTime) is running. Swipe downwards from top-right of the screen:

Third-party Apps, that support Centre Stage (e.g., Zoom and Webex) may have settings within the individual App.