I am positive Apple has at least one Refurbishment Center in the US that handles these. As noted, most are 15-day returns so they just need their disks scrubbed and diagnostics run. If actual repairs need to be made, they would be done at that place, as well.
I just ordered a refurb 2020 iMac 5K and it was in UPS' Louisville depot in less than 24 hours so Apple likely has a West Coast and East Coast center since I have had other refurbs start at their Ontario, CA depot.