I was developing uwp apps for the past few years, now moving to winui3. It is my first project. I am unable to find the designer view for the XAML files. Is it not available in WinUI3? or something else?

Unforturnately Designer View is not supported for now in WinUI3 for XAML files. You can track its status on Github:
https://github.com/microsoft/microsoft-ui-xaml/issues/5917
https://github.com/microsoft/microsoft-ui-xaml/issues/3656
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