Expo

Expo is a framework and a platform for universal React applications. It is a set of tools and services built around React Native and native platforms that help you develop, build, deploy, and quickly iterate on iOS, Android, and web apps from the same JavaScript/TypeScript codebase.

This is basically the tool that can or cannot be used for creating a new React Native app. The official React Native (Getting Started) docs also describe it. So React Native app can be developed with or without using Expo. It all depends on the developer to choose the framework that makes it easy for him to code. Expo is the only platform that helps to work both on android as well as on IOS.

Expo is like the inner part of React Native framework. Expo lets web developers build truly native apps that work across both iOS and Android by writing them once in just JavaScript.