Since you have used Expo to develop your react-native app you can build the android app file by going to the project folder and running the bellow command in the terminal
expo build:android The building process will take around 5-10 minutes. IfWhen your build is done, expo build:status will give you your .apk file url. If not, it will say your build is still in progress and to try in a little bit later again later.
Also to build the iOS app you can run,
expo build:ios To build the iOS standalone app you will need to have an Apple Developer account, but for Android standalone app you don't need a Google Play Developer account.
For the above commands to work you need to have expo-cli installed, For this you can run the bellow command,
npm install -g expo-cli To build the iOS standalone app you will need to have an Apple Developer account, but for Android standalone app you don't need a Google Play Developer account.
More Info: https://docs.expo.io/versions/latest/distribution/building-standalone-apps/
If you don't have an Apple Developer Account. You can build the iOS standalone simulator build using the command,
expo build:ios -t simulator And then wait for build to finish then download and unpack YourAppName.tar.gz. Then, start the iOS simulator and run xcrun simctl install booted YourAppName.app to run the .ica file.
More info: Expo / React-Native, Is it there a way to test-run standalone iOS apps on a device/simulator?