AFAIK the Start Menu itself is still C++ and XAML however only the Recommended section is build with React Native [1]. Funnily or rather sadly, they seem to be quite proud of using it as seen in the video.
In the challenges section, they list a few challenges with React Native that they had to overcome. Interesting thing is that MS already has XAML, WinUI etc., that they built and control and don't havr those "challenges". I don't understand what the team got by using React Native compared to using XAML. Anyone from MS here who could provide some technical benefits for this decision?
Still in some cases you have to wait a few minutes to get working Start menu search or menu itself on a new not low-end device.