The Xiaomi TV Box S 4K (2nd Gen) with Google Assistant runs on the Google TV operating system, which is based on Android. Although Google TV offers access to the Google Play Store, the installation of non-officially supported apps such as SmartTubeNext (an alternative to YouTube) is not possible directly via the Play Store.

However, such apps can be installed manually by downloading the APK file and installing it on the device. However, this requires several steps and can entail potential security risks, as the installation of apps outside the Play Store is not monitored by Google.

It is therefore technically possible to install SmartTubeNext, but it is important to consider the risks and the need for root permissions or other workarounds.