Is the inner cushion spongy, and covered in a sort of whitish wadding and stockinette? Then your cushion is foam, probably something similar to our Reflex foam( stockinette is a thin loosely knitted gauzy material - probably laddered, the laddering doesn’t matter at all- just watch it doesn’t catch in the zip!
Is the filling white fluffy stuff in a cover which looks a bit like white J cloth? The cover will probably have channels in it like a duvet, that will be Fibre. The substance in these channels may be quite solid when old, don’t be fooled, it almost certainly isn’t a species of foam, it’s just the fluffy fibre which is so matted with use that it has turned into felt.
Similar construction to (2)but the case be woven cotton, not a non woven synthetic, this is probably feather or a feather/fibre mix. It will have little fluffy spikes of feather poking through by now, and if it’s pure feather, the outer cover may be swimming with loose, broken feathers as well!