I don't follow the section in the wwdv video on http3
https://developer.apple.com/videos/play/wwdc2021/10094/
From about 07.00 onwards for a couple of minutes, the presenter says URLSession won't use http3 unless the server advertises it. Then he uses instruments to show the app is using http2 - BUT the server is advertising http3. Then he says the fact the server advertised http3 will be remembered for future sessions.
Why does the app use http2 for the first session even though the server advertised http3? Does that mean the first time any app connects to a http3 server it'll actually be http2 and only http3 for subsequent connections?