Then there are layers that decide how to render. Regular React has a renderer that accepts HTML tags spits out HTML for the browser. React Native has a renderer that accepts more abstract tags (<View>, <Text>) and spits out native views for apps. There are other projects that make renderers for making PDFs, animations, etc.
React Native for Web is basically a renderer that accepts the abstract tags React Native uses (<View>, <Text>) and spits out HTML. React Native does not really help you directly port a web app to native, but React Native for Web is designed to directly port a native app to the web.