This method does not need to be a root user or depends on paid applications, it is the only method that works, I did it with a Samsung with android 4.2, later versions I believe that from android 5 onward already allow you to install apps like whatsapp directly in external memory. But if not you can also follow the Guide.
How to install applications DIRECTLY on the SD card
With a single “command” you can configure your cell phone to download applications directly to the external SD card, if you no longer have internal memory available.
By default or factory, an Android device installs applications and games in the device’s built-in storage. When this memory is full and there is not enough space, the easiest thing is to pass the games and applications to the SD card. Although you can also force the installation of applications directly on the external SD card, with the help of a PC.
Prepare the PC: This procedure is done with the help of a PC. There you have to install the ADB and FastBoot software on the computer and the device drivers (if it is a Windows PC). For the former, the most practical thing is to install the ADB and FastBoot Minimal, and for the latter, the universal ADB driver.
Prepare the phone: As a final requirement, you must activate debugging by USB on the Android device.
How to install Play Store applications on SD card
|Command to configure the installation of applications on external SD memory
Press Enter. The serial number of the connected device should appear. Then write the following and press Enter:
adb shell pm set-install-location 2
Once this is done, the phone should install the Google Play Store applications directly on the external SD card. To verify that this configuration was successful and the installation is being done in the SD, execute the command: pm get-install-location. It should show the number “2” as a result.
If a message of the type “devices unrecognized” appears, what you have to do is go to developer options and mark where it says Revoke USB debug authorization and then restart the device.
I do not want the APPS to be installed in the SD
The previous change is reversible and necessary if some apps installed in the SD do not work as they should. Sometimes an error can be generated when heavy or large games are installed from the Play Store.
In order for the phone to reinstall applications in the internal memory (as it normally works), type the following command and press Enter:
adb shell pm set-install-location 1