In the 1920s, the use of reinforced concrete begins to become more common. The steel grids inside the hardened concrete give the structures stability. The grids are arranged in shells. The liquid concrete is sprayed into the shells. Afterwards, the concrete dries and hardens. The shells and grids can basically have any shape. This gives the architecture its charm, as roofs, pergolas or other parts of buildings seem to defy the earth's gravitational pull and do not have to be supported by pillars or something similar. Architecture before World War II did not yet make full use of the range of reinforced concrete structures. It is mostly oriented towards cubic forms to design the houses. Only occasionally experiments with this material are implemented. After World War II, buildings are created that apparently defy the laws of physics. One of the pioneers for that kind of architecture is Oscar Niemeyer, who uses reinforced concrete to construct, for example, the upper end of buildings much wider than the foundation and ground floor, or chooses very sweeping forms that only became possible through the use of reinforced concrete.
A detailed explanation and history of reinforced concrete can be read here.