That’s a simple connection between two steel beams, made in the 1940s. The beam on the right ends at and is supported by the beam on the left. The connection angles – the one we can see, and the one on the opposite side of the supported beam – were shop riveted to the supported beam. That beam came with the angles in place to the construction site, where bolts were used to connect the angles to the supporting beam.
This is all ordinary and similar to what happens today, except now we’d use shop welding or bolting rather than riveting. But this sequence is the end result of decades of development of steel technology, including analysis and design of the connectors, the use and review of shop drawings, and the analysis of such details as the flange copes.
It’s easy to take a fully-developed technology for granted.