You cannot use AirPlay to mirror your screen from a MacBook Air to an iPad. To use an iPad as a second screen, you would use the SIDECAR feature available in System Preferences of the macOS.
Unfortunately, you will still not be able to use SIDECAR because it requires a MacBook Air 2018 or Later. Yours is a 2017 MacBook Air. Your iPad Pro qualifies.
Back to AirPlay in Big Sur. AirPlay is known as Screen Mirroring and you can locate that in the MENU BAR at the top of your screen. You can AirPlay to a Smart TV through the Apple TV device OR directly on an AirPlay capable smart TV.
Axel F.