According to this https://github.com/microsoft/fluentui repository, there are many UI library from fluent UI.
@fluentui/react-northstar library is used to develop Teams App development. but I have no idea why another similar Fluent UI React Components v9 library is there.
What library is recommended for Teams app development ?
This link should answer your question. In short, React Components v9 will eventually supersede react-northstar.