To use Sidecar wirelessly, place your iPad to the left or right of your Mac. Then click on the AirPlay icon in your menu bar.
If you have an AirPlay icon, you'll see Connect to in its dropdown menu, with the name of your iPad next to it. Click on that, and you're done.
Depending on how your Mac is set up and whether you've ever used AirPlay before, you may not see the icon, though.
If you don't, go to any open Finder window. Press and hold on the green traffic light icon. You'll see a new dropdown menu that includes options to rearrange the window on the screen —or to Move to your iPad. Do that, and Sidecar is working.
Also, having done this, you'll now have the AirPlay icon in your Mac menu bar and from now on can use it to connect and disconnect to this iPad.