How is concrete poured for very high floors of skyscrapers?

We have special kind of pumps called ‘concrete pumps’ which are designed to create extremely high heads and thus pump concrete to higher levels through metallic pipes which can be manually primed through flange joints to reach desired heights. Depending upon the height of casting site, a pump of proper capacity is chosen. The pump operates on a reciprocating principle.
However, when concrete is pumped to large heights, seggregation of concrete is evident which could be overcome by staged pumping. Moreover, pumps can’t withstand the resistance offered by long vertical column of concrete present in the pipes, which also necessitates staged pumping.

The concrete is pumped up to the point of placement with a pump, usually diesel powered. It’s pumped through pipe that must be cleaned after each use. Pipe is added as the building goes higher. Placement is usually assisted at the point of placement by an articulated boom, with a flexible hose at the end. There are usually two operators, one on the ground to mind the pump, and one on top of the building to operate the boom.

The concrete usually losses some slump on the way up the pipe. Concrete mixes for pumping are carefully chosen to survive the pumping process with sufficient workability and the required strength.

There are various opportunities for mishap: the pipe can clog or burst, the pump or articulated boom can fail, or air in the line can cause the boom to kick dangerously, but it would be difficult to imagine placing concrete for a high rise without pumping. Placing concrete with a crane bucket is possible, but slow under the best of circumstances.
That’s usually how it’s done around where I work. Construction is a very regional business.