React, react-native and react-native-web are really interesting. Classic react (for the web) was built with a plugin based rendering system. Classic react's renderer targets HTML and the DOM. React-native swaps out the renderer for one that renders to native elements (for example, instead of a <div> you would use a <View>, which then renders a native "box" using system primitives). They are both running the same react code, just with different renders. There's a whole system in react-native to be able to wrap native APIs and access them via javascript, but at the end of the day, both systems are running the same react (just different renders).
React-native-web is basically a classic react component library that aims to be 100% compatible with react-native. You can literally do "npm install react-native-web", then "import {View, Text} from "react-native" and just start writing react-native code in a classic react-project. It's nearly feature complete, and pretty incredible. Really the only downside is that (obviously) react-native-web can't use those 3rd party libraries that use that "native API wrapper" features of react-native. That being said, a lot of packages are starting to offer react-native-web compatibility by writing web-specific "polyfills" for the native components.
React-native-web is basically a classic react component library that aims to be 100% compatible with react-native. You can literally do "npm install react-native-web", then "import {View, Text} from "react-native" and just start writing react-native code in a classic react-project. It's nearly feature complete, and pretty incredible. Really the only downside is that (obviously) react-native-web can't use those 3rd party libraries that use that "native API wrapper" features of react-native. That being said, a lot of packages are starting to offer react-native-web compatibility by writing web-specific "polyfills" for the native components.