These are pre-assembled single coil springs on a metal bar that is attached to the chair frame with nails or screws. The coils are larger at the top and smaller at the bottom where it is fastened to the bar.
Normally, there are 3 or 4 coils on a bar. Bars may be available in 21 to 24” lengths to fit the opening of the chair seat frame.
Older textbooks sometimes referred to them as a “spring metal bar”. They are not very common any more. And they are difficult to locate if you need to replace any in a reupholstery project.