yeah idk about the macos app personally but i can say that the windows app is definitely electron. at least last time i checked.
by 'react native' i'm guessing you're referring to RNW, although a port of their iOS app is possible
Facebook had previously just used a port of their iOS for their Windows app too.
I think we need a new acronym. RND (React Native Desktop)? Microsoft is doing React Native for macOS alongside of React Native Windows. The macOS port is a little bit behind the Windows one, but actively being worked on by Microsoft and Facebook employees, afaik.
by 'react native' i'm guessing you're referring to RNW, although a port of their iOS app is possible Facebook had previously just used a port of their iOS for their Windows app too.