Rc transmitter doesn't have range of 2km but dji said "8km range". How dji can say "range of 8km"
how it is possible?
Which rf chip is used by dji?
which frequency hopping technique used in dji?

There are some long range systems available to purchase separately. Dragonlink provides long range video transmission. 433MHz transmitters provide long range. However, DJI uses proprietary occusync system. Not sure what chip they use.

RC transmitters does have range more than 2 kms. The standard basic cheap stuff has a range of just 1.5 km. I haven't researched about the RF chip which dji uses. You could try doing it yourself. But, from what I know already, you can Increase the range of a transmitter by increasing the wattage of the transmitter. That is, when power increases, range increases. So, a 1 W transmitter in the same antenna setup will have twice the range than a 0.5 watt transmitter. Besides, DJI drones are capable of a range of at least 14-15 kms los. They have set a barrier for the range to not exceed that distance.

@AL HAM SHARI
You are right. By increasing power, we can increase range.
But dji information is different.
https://www.dji.com/mavic/info
950mA @ 3.7V
Battery   2970mAh
DJI controller are small & i think weight of controller is not 1 kg.
So, how dji controller can transmit over long range.

That's exactly what I said. 950 mA = 0.95 A . ie, .95 Watt per volt. That's nearly 1 watt.

but voltage is 3.7v
watt = 3.7 v x 0.95 A = 3.515
3.515 watt is not okay for 10-15km range.

Firstly u have got wrong information that RC transmitters have not more 2KM range. Its a blunder. There are many radios which have very good range. However it also depends few other factors such as tx power, receiver sensitivity, distortion, modulation type etc. Do note that sensitivity is also very important.

Coming to power consumption of transmitter: Power consumption depends on antenna power output, bitrate transmission. For tx bitrate is generally low compared to vtx which reduces the power consumption.
We cannot directly comment if 3.5W power is sufficient for 10-15KM range as it depends on above factors.

DJI specified range may be without any distortion and in open air or LOS.
Their RF chips are not off the shelf parts to buy. They are generally custom made chips or some OEMs might be making them for DJI.
Coming to video transmission, unlike analog it is completely digital which makes transmission lossless and at higher bitrates.

Besides, with a 100k price tag, you'd want to offer that.

I didn't understand the this.

I mean , Most DJI drones cost at least 1 lakh to maintain including fly more package. So, at that rate they should do something amazing.

I don't say they are overpriced. I will breakdown the price of components. Lets say Phantom 4 Pro V2.0 which costs $1499.
The quadcopter itself will cost you 300$ to get you flying.
However for the stability you need to pay the premium for flight controller.
You also have those extra sensors for collision avoidance and improving stability.
You again need to spend extra bucks for that radio control range.

Now comes the hero the camera. Add on camera will really increase the cost because of the CMOS sensors, gimbals, cabling and extra hardware involved for on board processing. This needs high cost hardware and good engineers to produce them.

Now one more hero the downlink(contains both transmitter and receiver on radio) which is necessary to transmit HD footage and receive the same over few miles. This will cost really much not just in terms of hardware but also in terms of software and research involved.

One more important thing is they are playing it monopoly. There is no good competitor with that many product lines and with similar quality as of now. So they charge what customer is ready to pay for their need.