Must Follow Android First:
<- ANDROID CONFIGURATION

iOS CONFIGURATION:

Step 1:

Open myapp.workspace

xed -b ios

Step 2:

Duplicate myapp in Target section by right click and rename them production and development.

Step 3:

Go to Manage Schemes...:

Rename the myapp copyies to development/production

Step 4:

Go to Edit Scheme...:

Go to Build
Pre-actions
Select scheme(like development in screenshot)
Click + to add 2✌🏻 "New Run Script Actions"
Select scheme(like development in screenshot in Run script also)

Add this in First Run Scripts:

# Type a script or drag a script file from your workspace to insert its path.
rm "${CONFIGURATION_BUILD_DIR}/${INFOPLIST_PATH}"
echo ".env.development" > /tmp/envfile

Add this in Second Run Scripts:

# Type a script or drag a script file from your workspace to insert its path.
"${SRCROOT}/../node_modules/react-native-config/ios/ReactNativeConfig/BuildXCConfig.rb" "${SRCROOT}/.." "${SRCROOT}/tmp.xcconfig"

DO FOR PRODUCTION SIMILARLY with .env.production

Step 5:

Select scheme(like development) and rename Info.plist File

Select new generated myappInfo.plist(1) and rename(2)

Also in Bundle display name in .plist put $(DISPLAY_NAME) or App name as per environment.

Step 6: iOS Change App Icon and App Name

Change App name as per scheme in Display Name/ also you can change in Info.plist of 'Bundle display name':

Change App Icon: First, add/duplicate the new icon set in Images.xcassets and replacing with specific icons.
Then, in Xcode, navigate to the app icon settings and select the new icon from the available options.

Finally, properly clean project and RUN.

Step 7:

In Podfile make these changes:

    # react-native-config
    pod 'react-native-config', :path => '../node_modules/react-native-config'
    # For extensions without React dependencies
    pod 'react-native-config/Extension', :path => '../node_modules/react-native-config'
    # react-native-config

Also add these:

  target 'development' do
    inherit! :complete
  end
  target 'production' do
    inherit! :complete
  end
//  target 'myapp_mappTests' do
//    inherit! :complete
//    # Pods for testing
//  end

Use these command to run app:

"scripts": {
    "iosAppStart": "yarn ios:dev",
    "iosAppStartProd": "yarn ios:prod",
    "ios": "yarn setDevelopment && npx react-native run-ios",
    "iosNew": "yarn setDevelopment && npx react-native run-ios --simulator='iPhone 16'",
    "iosSE": "yarn setDevelopment && npx react-native run-ios --simulator='iPhone SE (3rd generation)'",
    "ios:check": "react-native run-ios -h",
    "aos:check": "react-native run-android --list-devices",
    "lintFixAll": "eslint 'src/**/*.{js,jsx}'",
    "prettierFixAll": "prettier --write 'src/**/*.{js,jsx}'",
    "fix:lintPrettier": "yarn prettierFixAll && yarn lintFixAll",
    "cleanCache": "npm start -- --reset-cache",
    "start": "react-native start",
    "killAllBundler": "killall node",
    "adb": "adb reverse tcp:9090 tcp:9090 && adb reverse tcp:8081 tcp:8081 && adb reverse tcp:3000 tcp:3000",
    "installAll": "yarn && cd ios && pod install && cd ..",
    "cleanGitIgnoredFiles": "rm -rf .jso node_modules package-lock.json yarn.lock && cd ios && rm -rf build pods .xcode.env.local Podfile.lock && cd .. && cd android && rm -rf build && cd app && rm -rf build && cd .. && cd .. && yarn installAll",
    "setDevelopment": "ENVFILE=.env.development",
    "setStaging": "ENVFILE=.env.staging",
    "setProduction": "ENVFILE=.env.production",
    "aos:dev": "yarn installAll && yarn setDevelopment  react-native run-android --mode=developmentdebug --appIdSuffix=myappdev",
    "aos:dev-release": "yarn installAll && yarn setDevelopment react-native run-android --mode=developmentrelease --appIdSuffix=myappdev",
    "aos:prod": "yarn installAll && yarn setProduction  react-native run-android --mode=productiondebug --appIdSuffix=myappprod",
    "aos:prod-release": "yarn installAll && yarn setProduction  react-native run-android --mode=productionrelease --appIdSuffix=myappprod",
    "aos:clean": "cd android && ./gradlew clean",
    "aos:DR-apk": "yarn setDevelopment &&  yarn aos:clean  && cd android && ./gradlew assembleDevelopmentRelease",
    "aos:PR-apk": "yarn setProduction &&  yarn aos:clean  && cd android && ./gradlew assembleProductionRelease",
    "aos:DD-apk": "yarn setDevelopment &&  yarn aos:clean  && cd android && ./gradlew assembleDevelopmentDebug",
    "aos:PD-apk": "yarn setProduction &&  yarn aos:clean  && cd android && ./gradlew assembleProductionDebug",
    "open-apk": "open ./android/app/build/outputs/apk/",
    "aos:dev-apk": "yarn aos:DD-apk && yarn open-apk",
    "aos:prod-apk": "yarn aos:PD-apk && yarn open-apk",
    "uninstallApks": "cd android && ./gradlew uA && cd ..",
    "aos:dev:release-apk": "yarn aos:DR-apk && yarn open-apk",
    "aos:prod:release-apk": "yarn aos:PR-apk && yarn open-apk",
    "ios:dev": "yarn installAll && yarn setDevelopment && react-native run-ios --mode=Debug --scheme \"development\"",
    "ios:dev-release": "yarn installAll && yarn setDevelopment && react-native run-ios --mode=Release --scheme \"development\"",
    "ios:prod": "yarn installAll && yarn setProduction && react-native run-ios --mode=Debug --scheme \"production\"",
    "ios:prod-release": "yarn installAll && yarn setProduction && react-native run-ios --mode=Release --scheme \"production\"",
  },

Usage of .env values in Native side: