Native and Ionic framework lies in the fact that they propose dissimilar approaches towards the application development. Ionic helps you for example to build a typical hybrid app that performs rendering in WebView for Android or UIWebView for iOS. It is typically used for creating so-called SPAs (single page applications).
In case you need the API platform you will have to apply additional plugins - the abovementioned PhoneGap or Cordova. Unlike the Ionic framework React Native lets you build a mobile application with the standard UI blocks. The application on RN uses the templates mobile app development service designed specifically for each platform. It is mostly used for native programs as its design is as close to the native's as possible.
React Native involves the native rendering APIs in Objective-C or Java for iOS and Android respectively. With it the application renders with real UI components not WebViews. Ionic and React Native have a different technology stack. Ionic developers create one application on top of a browser and can use all the npm packages available for React.js. And React.