The Companion App run natively on an Android or iOS device. Current version of the app is 2018.06.06. You can download them on each store:
Since 2018-06-06 upgrade, the Companion App has the exact similar experience as the web interface available http://player.inout.tennis. They wrap the exact same content. Browse the Web Interface page to understand the different features of the web interface, hence the Companion App.
The only difference (and advantage) between Android and the web interface is that the Companion App for Android automatically switches between the In/Out Wi-Fi Hot-Spot and your default Internet Wi-Fi router. Therefore, on Android, you do NOT need to visit the Wi-Fi settings to associate your phone or tablet to the In/Out device. The Companion App does it for you. On the web interface (any browser such as Chrome, Safari, Microsoft Edge) or with the iOS Companion App, you need to visit the Wi-Fi settings and associate your phone to the relevant Wi-Fi network, aka. either the In/Out device or your usual Internet router.