Let me post an answer with some new information.
What is the difference between Windows App SDK and WinUI 3?
The Windows App SDK is the overall development platform for modern Windows desktop apps. It provides APIs, deployment tools, and system integrations. WinUI 3 is the UI layer within Windows App SDK. It gives you modern controls, styles, and fluent design for your app interface. So, WinUI 3 is PART of the Windows App SDK.
And what is the best way to develop Windows 11 apps? Still UWP? Or Windows App SDK? Or WinUI 3?
UWP is still supported but largely in maintenance mode. For new apps targeting modern Windows 11 features, I'd recommend WinUI 3 with the Windows App SDK. If you're looking for a mature ecosystem with extensive community tooling and legacy compatibility, WPF remains a solid choice.
This post should help.
FYI: WinUI 3 is moving toward open source, which may drive more community involvement and faster progress.
https://github.com/microsoft/microsoft-ui-xaml/discussions/10700#discussioncomment-13980717
UWP dont have round corners thats correct?
Yes and no. UWP apps use the older WinUI 2 library, which got rounded corner support starting with Windows 11—but it’s limited. Your app window may get the rounded look automatically, but individual controls (like buttons) won’t unless explicitly styled or updated with newer versions of WinUI 2.
And Mica is not only availabe in WinUI 3?
I haven't tried it but according to this doc, you can use Mica on UWP.
Windows.UI.Xamlnamespace. The Win UI 2 implementation lives in theMicrosoft.UI.Xamlnamespace.