To generate the .APK for android just run the command:
gradlew assembleRelease
Inside the android folder…
But, and in IOS how do I do it?
>Solution :
First of all to build an iOS Application you will need a macOS Device, with Xcode installed.
To Build a Debug App enter npx react-native run-ios in your console.
To Build a Release Version use the following command npx react-native run-ios --configuration Release
The following Guide in the React Native Documentation explains how to build a Release Version in Xcode.
https://reactnative.dev/docs/publishing-to-app-store
Please note that without a paid Apple Developer Account your iOS Distribution is very limited. i.E you cannot install the App without Physical Access to the device. You won’t just get an Executable for iOS.
More information on iOS Distribution can be found here https://help.apple.com/xcode/mac/current/#/dev067853c94