What is ReactJS?

ReactJS is a JavaScript library that can be run both on front-end web (in the browser) or on a server (Server Side Rendering). It allows for building user interfaces and web applications.

It uses JSX as its primary syntax, which is a HTML+JS+CSS combination developed by Facebook.

It was invented so JavaScript can manipulate the website DOM faster using the virtual DOM, which uses a smart diffing algorithm to only update the UI when needed, and doing so in batches, resulting in a more performant UI.

What is React Native?

React Native is a mobile applications development framework built on top of ReactJS, that allows you to build performant, native mobile applications by writing code that is very similar to pure React.

When compiled, it turns the Javascript & JSX code into native iOS/Android app components (also, see React Native for Windows and Mac and Expo Web).

Are React Native and React interchangeable?

For short - no.

Firstly, you cannot use React Native without also using React. Secondly, React Native is not a tool that can be used on the web (even if third party efforts do exist) or in SSR.