This page describes how APK expansion files work in the context of a Unity Android application.
Unity can automatically split an application into a main APKThe Android Package format output by Unity. An APK is automatically deployed to your device when you select File > Build & Run. More info
See in Glossary and a single APK expansion file that uses the .obb file extension. In Unity, this process is called splitting the application binary. The APK expansion file this process generates is the main APK expansion file and if you want to create the patch APK expansion, you must do that manually.