[daviddever23box]

React Native, absolutely - as with Office 365, using WinUI 3 as the underlying UI layer.

[madeofpalk]

Running a full javascript framework in a javascript runtime.

[kristianp]

When did office start using React Native? Office has been around well before the creation of javascript, let alone react.

[p_ing]

I'm not sure that it uses React Native, but modern add-ins for Office are JS, so any injected modern add-in will be using JS rather than COM/C/C++. This gave add-ins portability between all supported Office platforms, increased security, and made administration much easier.