It uses React Native, via https://github.com/microsoft/react-native-xaml. This is probably mixed with "native" XAML components, but it's still firmly in the "buggy React app" category in my book: a framework that works well with React probably has the same design flaws that lead to React-esque crashes.

It's very difficult to find information about this online: https://news.ycombinator.com/item?id=46471701 is the only thing that remotely smells like a secondary source, with most of the rest of the search results being obvious ChatGPT output. (That, too, is Microsoft's fault.)