Replacement of iPhone after repair, can I keep my old?

So I had my iPhone 14 Pro exchanged for a new one at an official apple service point due to them estimating the repair itself beeing too big of a hassle and therefore not worth it. So they basically replaced „all the parts in the old phone“. So my question is, can I keep my old „replaced parts“? And if not why not? I payed 899€ for the service so I effectively payed for the replacement phone, so I dont see why I should let them keep my old one.