Does the UI layer change between iOS and Android in React Native?

I am just exploring react and have couple of doubts. I don't see common UI components for react native apart from scroll, flexbox, input list in the documentation. So if I am writing for iOS will the UI part be different from that of Android? If I am having a web app written in react, can I reuse it (no div, img etc for react native)? So is there any possibility of reusing the web UI to native UI? So how exactly is it cross platform? I can reuse the logic only, right? Thanks.