iOS devices refurbs

Are you talking refurbs or service replacements? That's two different things. Refurbs are products that were returned, etc. and are repacked and sold on the Apple refurb store. I think (but not 100% sure) that these items retain their original serial number, but they do have their original sale date reset so you get full warranty.

Service replacements are manufactured out of new & used components and get a new serial number. You can't buy a service replacement, they're just used for exchanges in lieu of repair.