react native build release from .apk to .aab, how to send the app to the clients?

It using for different android devices installation by google play store. that's why you can use apk build for manual installation.

You must use the below command for the testing build.

$ cd android
$ ./gradlew assembleRelease

As mentioned repeatedly in many places .aab file is efficient. But we need .apk for regular task (except PlayStore).

So:-

1. Create signed output of Android project Link
2. Use command $ npx react-native run-android for debugging.
3. Finally $ npx react-native run-android --variant=release Link, to create release version, which is created at android/app/build/outputs/bundle/release/app.apk
4. Go crazy

Just for the information. If you upload the AAB to Google Play instead of the APK, it will lead to smaller download sizes. AAB lets Google Play create different APKs to different devices, based on the device screen density, language, and CPU architecture. So, if you're able to create AAB's, use them over APK's.