I think this was shot with a DJI S800. The RC link to fly it is usually 2.4GHz. A separate link, often at 5.8GHz, is used to send the camera feed back to the ground to help the pilot or compose the shot, or both.

Good eye ID'ing the distinctive DJI arms on the hexacopter.

The convention seems to be 2.4Ghz for telemetry back the to base-station or radio display - and something lower in the UHF band like 433 or 900Mhz for actual control.

Mainly because Zigbee/802.15.4 radios are readily available in 2.4Ghz and 433Mhz goes a lot further (and through more things) by comparison at 1W.

[/sitting next to 3 hexacopters at the moment]

What kind of transmitter are you using for manual control? I assume you aren't using a ground station all the time...

I only ask because most new 'brand name' (JR, Futaba, Airtronics, Robbe) tramsitters seem to be 2.4Ghz. I know FM radios are still available on 36 and 40 Mhz, among others, but I haven't seen many used for a while with new aircraft.

UHF transmitters can be rigged on a RC transmitter as a module or through the trainer port, depending on the radio. DragonLink and EZUHF appear to work this way. (I haven't used any of these.)

Correct. The EZUHF is my preferred transmitter.

A simple DJI Phantom will easily get to that altitude. Slight modifications would be required - possibly a different receiver and remote control to ensure the range, and an FPV system (camera and transmitter on the copter, and a receiver with a screen or video goggles on the ground). Altogether easily done in under $1000.

Control is real-time, just like any other flying model aircraft or multirotor. For flying at that altitude, I'd use FPV - meaning you'd fly using the camera image to pilot the copter viewed through a screen or even better, video goggles, as opposed to looking at the model from the ground.

Be aware though that at this altitude, you're often in controlled airspace, which is troublesome for model aircraft in most cases.

I may have just started looking at building a tricopter with FPV. This is going to be an expensive hobby, I can feel it. Satisfying, though!