An important distinction that often gets lost is that React itself is an abstract, standalone layer above web and native. It handles deciding what to render and when. When you learn the concepts of React, you're learning this layer.
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.
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.