[Homer]

Taking a stab from WW2 US practice.

High Value prisoners (crypto/commo, special weapons, leadership, technicians, sub/bomber crew) would be a commodity and probably get a special high value camp for extended exploitation. This was the case with the camps at Fort Hunt or Pine Grove Furnace. Perhaps the initiation of tactical nuclear warfare causes the European allies to evacuate this class of prisoner to the US and Canada, swelling what’s already there.

Regular POWs would likely be backhauled through the supply system to the theater logistics heads- Antwerp and Bremerhaven on the central front or bandar abbas in Iran for example. At that point they could go back to the US and Canada to free up host nation forces and resources. This would require a legal opine as POWs are legally the responsibility of the country capturing them and not to be change custodians. Perhaps a broader definition of NATO as an entity under common command similar to the broader definition of British Empire that allowed UK captures to be sent to Canada in WW2. Otherwise, each nato member establishes their own camps- that would really strain some states.

POWs captured in austere theaters, for example in CENTCOM, Southern Europe, Norway, or at sea may be sent directly to conus or camps by logistic backhaul. This happened historically with Afrika Korps and some Italian POWs as there was less strain in holding them in North Africa.

Just my thoughts