I think that depends a lot on the industry. There's a lot of evidence Amazon is recycling "unopened" returns into the new stock pool because people are getting sealed items with their contents replaced because it turns out it's not that hard to shrink wrap items well enough to convince an overworked return intake employee.

Some also gets diverted into return resale stores where companies like Amazon will sell a 3rd party a pallet of miscellaneous returns they then sell.