The ipad shows up just like another monitor to the Mac. You can move your mouse into it and drag windows to and from it. The ipad seems to just be a dumb display (and touch input) when connected. I believe it requires a USB-C connection.
Ah, that's great! I use Duet (paid app) for this at the moment but it's disappointingly flaky - it seems to only work sometimes and when it does, you'd better not touch anything unless it resets and stops working again. Worth the setup effort for longer sessions but hardly practical for shorter ones on the go (which let's face it, is the main usecase). Having this native so it 'just works' out the box would be amazing.
But I don't understand if it's true second screen, i.e. a native replacement for what the Duet app does, or more like a second screen 'portal' for specific apps that implement that functionality.