It's a steel deck with concrete poured in - I think !
"A steel deck with a poured concrete floor, known as a composite floor system, uses corrugated steel sheets as permanent formwork and reinforcement, creating a strong, cost-effective, and fast-to-install structural slab by combining the concrete's compressive strength with the steel's tensile strength. The deck acts as a working platform and holds wet concrete, while shear connectors (like headed studs) bond it to the steel frame, forming a single, load-bearing unit after curing, common in commercial and multi-story buildings."