Main and patch expansion files must be in a particular location on the device for the application to read from them. If you Build and Run your application, Unity installs both the APK and the main APK expansion file on the device.

If you instead want to install the application manually, or want to install a patch expansion file, you must use the utility to copy APK expansion files into the correct location on your device. For information on how to do this, refer to Testing file reads.