This has been making the rounds on Twitter and the story never dies despite being essentially false. (Backed by “I hit Start and got a CPU spike”)
Just the “Recommended” section - the lower frame showing M365 files & frequent apps - is built with React Native[1]. And, AFAIK, it uses Microsoft’s React Native for Windows that renders real XAML components, so it’s not just a masquerading WebView or something.
The Start Menu itself is still C++ and XAML. (And I think WinUI 2, i.e. the system-included variation of WinUI) It’s not just all a web thing or a JS-backed React Native app.