Consistent cross platform: This is actually the one thats probably most difficult with react-native. Since unlike electron or Flutter, React-Native uses native UI for each platform. This means you can achieve full parity with native apps in terms of the UI. But all the platforms have different capabilities so since react-native can take full advantage of all the platforms, and isn't lowest common denominator, there are differences between the rendering / support across multiple platforms.

Can the native UI framework do multiple windows? - Then React-Native can do multiple windows.

Rendering to a windows that isn't owned by the framework is what React-Native refers to as Brownfield apps. This is absolutely supported. For instance, Microsoft Office is using React-Native for some parts of their UI in Android, iOS, Windows and macOS. And the Office window is not created or owned by react-native.