Under the cushion of this sofa is the part nobody sees and everybody feels: the sprung seat platform.
Webbing is stretched across the seat frame first, interlaced and tensioned by hand until it rings when struck. Coil springs are then stitched to that webbing at each intersection, and each spring is tied to its neighbours and down to the frame with cord - eight ties per spring, so that a spring cannot lean, only compress.
The tension in those ties is not uniform. The springs at the front edge are tied down harder than those at the centre, because the front edge takes the load of someone perching on it and needs to be firmer, while the centre should give. Set them all the same and the seat feels either hard everywhere or soft everywhere; graduate them and it feels correct in both places.
The whole platform is then covered and the cushion sits on top of it. A sofa built this way takes several days longer than one built on a fixed foam base, and it is the reason a good sofa is still comfortable in twenty years while a cheap one is not comfortable in five.