Android has an APK installer built in. Opening an APK file launches the installer and installs the application, just like opening an MSI file on Windows launches built-in Microsoft Installer and installs the application.
Google have gradually added impediments to this over this years, such as a requirement to toggle a checkbox in the settings to enable installation, and later some prompts about letting Google scan the package, but calling the system's built-in application installation mechanism "not primary" is absurd.