React and React Native are separate tools, and they don't play with each other. You actually need to build two separate apps and you don't get much of any re-use. For example, you can't use Material UI for React in React Native, and you can't build a React component library and just pull it into React Native.
So if you want a fully cross platform React experience, you need React Native from the get-go and to compile it for web when you want web.
So if you want a fully cross platform React experience, you need React Native from the get-go and to compile it for web when you want web.