Is that true that react code can be used in web/mobile/iOS/Android?

I just started to learn React and don't have a full clean image of how React works.

I was just wondering if I provide server API, and wrote my website(frontend) in React, without modification or with little modification, I can use this code in my PC website/mobile website/iOS App/Android App?

That is the purpose why I started to learn React, I want one code running in PC website/mobile website/iOS App/Android App calling same server side API and show same UI to the users.