How exactly works the quantum communication done by China? This video and this article announced that spacial quantum communication were finally done using satellites. 

As a recap, when two particles are entangled, if a measurement is done to one of them, the appositive value results in the other particle. Nevertheless, traditionally is believed no practical communication is possible to do using this approach because the data is random.

The video says that the way the dealt with the problem is that some information is send trough the quantum channel but extra information is sent also using conventional information which is complemented with the quantum information to get the complete message.


How exactly it works?
What the extra information sent in conventional channel is about?
Is this extra information smaller than the original message?
If not smaller what's the point of using the quantum channel and not just sending all the message in the conventional channel?


Finally they say that it offers unhackeable security. Is that statement valid in any way?
I watched the video with the sound off, but the giveaway are the lasers. It's simply(!) the extension of quantum bit transmission to orbiting satellites, rather than via fibre optics.  All the same principles apply, although it's hard to keep a physical lock on the satellite.  Clearly this becomes impossible when the satellite disappears beneath the horizon.  

There is a rule of thumb that says general media + tech = 0.  The general press are hopeless in trying to convey a technological story.  What the Chinese will have done is to transmit entangled photons to the satellite.  They may have then send another set of entangled photons back to the earth station.  This allows transmission of data via statistic analysis, and the BB84 protocol is a common technique for this. The Chinese may not have got as far as this protocol.  It would be sufficient to prove entanglement between the earth station and the satellite. This would be the real advancement in this item.

This answer details how a quantum network works.  In summary, keys are generated via quantum RNGs and transmitted via photons.  Transmission is akin to a one time pad (OTP), but a bit cleverer as there is a great deal of noise within the transmission.  That's why the key exchange is unbreakable.  Those keys are then used for conventional encryption over conventional wiring.  With a  satellite, that conventional encrypted transmission could be achieved via lasers, just minus the glass fibrery bit.


  If not smaller what's the point of using the quantum channel and not just sending all the message in the conventional channel?


That's a good question.  Currently the key exchange is done via OTPs.  It's low baud, so a conventional channel is used in an office setting for the Gigabit throughput.  ID Quantique only manage about 10 KBits /s currently.  It is conceivable that when this technology is miniaturised to portable size, military battlefield communication could all be performed via pure BB84, without bothering with the conventional encryption. And it would all be unbreakable, whilst currently the conventional encryption might be broken one day.  

But it might be jamable and susceptible to MITM attacks.  Another almost coincident laser directed at the satellite receiver could swamp it.  This is speculation at this stage, but someone's working on it, I'm sure.