Central is a building made out of concrete, one of the most common construction materials that can be used in a vast range of applications such as dams, foundations and walkways. It's also one of the most economical and versatile construction material and essentially consists of cement, aggregates, water and additives.
To create a structure, concrete is poured into a mold so that it can take its shape. In this case, the mold is made out of plywood sheets supported by a metal scaffolding system. For each floor, the scaffolding system has to be assembled and disassembled. The plywood sheets are added on top. That's quite a bit of work!
There are many ways to build a concrete structure. In the case of Central, the large U-shape structure is being built in two sections. The north section (Part 1) was built first and the south section (Part 2) is currently being built. I imagine it will be done relatively soon. The structure of the north section was completed a couple weeks ago and the 6th floor of the south section is being poured. Building in sections allows other trades, such as plumbers, framers, and electricians, can start working sooner.
To build the structure, every so often a concrete truck comes on site to deliver the concrete. The concrete is hoisted to the appropriate level with a crane, one "giant bucket" at a time. Pumping the concrete would have been faster, but also more expensive. On each level, steel bars have been arranged in a grid pattern ahead of time in order to reinforce the concrete, hence the name "reinforced concrete". Concrete is very strong when you compress it but not as strong when you try to bend it, so the addition of the steel bars gives it more tensile strength. Most concrete structures nowadays require steel bars.
So the concrete is poured on top of the temporary plywood floor to cover the steel grid. It is then made smooth by the workers. The tall steel bars you see sticking out of the concrete slabs are used to reinforce the concrete walls which come next. After the floor slabs are poured, forms in the shape of boxes are placed above the slab to create the walls. Concrete walls will be present throughout the building for the elevator shaft and staircases for example.