Answers for "how to build apk using react native"

react native apk build

For windows, gradlew assembleRelease

For Linux, ./gradlew assembleRelease

react-native android build apk

cd android
./gradlew assembleRelease

react native release apk command

npx react-native run-android --variant=release

export apk react native

Before uploading the release build to the Play Store, 
make sure you test it thoroughly. 

mkdir -p android/app/src/main/assets
react-native bundle --platform android --dev false --entry-file index.js --bundle-output android/app/src/main/assets/index.android.bundle --assets-dest android/app/src/main/res
react-native run-android --variant=release

Export Apk -
cd android && ./gradlew bundleRelease -x bundleReleaseJsAndAssets

- Your AAB BUNDLE will be present in the folder
<project>/android/app/build/outputs/bundle/release

react native generate release apk

Prerequisite:
-----------------
react-native version > 0.57

How to generate one in 3 steps?
---------------------------------  
Step 1: Go to the root of the project in the terminal and run the below command:
---------------------------------------------------------------------------------
react-native bundle --platform android --dev false --entry-file index.js --bundle-output android/app/src/main/assets/index.android.bundle --assets-dest android/app/src/main/res

Step 2: Go to android directory:
---------------------------------

  cd android

Step 3: Now in this android folder, run this command
------------------------------------------------------
  ./gradlew assembleDebug
  
There! you'll find the apk file in the following path:
yourProject/android/app/build/outputs/apk/debug/app-debug.apk

Now you have your .apk file generated, install it on your android phone and enjoy!

Thank you.