Question about Xiaomi MI Box S

With the Android TV app, the smartphone can be used as a "keyboard".

You can also connect a normal USB mouse and keyboard or use the remote control via Android voice search. Both work quite reliably.
Depending on the app, the operation is somewhat easier and better designed. I use Zattoo and am very satisfied with the operation.

You can use the Android TV app as a keyboard, or you can connect any Bluetooth keyboard (e.g. Logitech keyboard). You can change channels via the selector wheel.

Depending on the app, the channel can be changed with the selector wheel, e.g. with Zattoo with a click up/down. If Bluetooth is active, you can use the box with the keyboard app, and so tedious typing of passwords etc. can be controlled by means of the selector wheel on the on-screen keyboard, entered in a very simple way on the smartphone.