First 748 with pallets/containers of them just arrived in Louisville, KY this AM. The supply chain on these (and most new AAPL stuff) is direct distribution from FOXCONN (or other manufacturer) straight from China (literally, it is boxed and labeled for end consumer right there/then and even pre-sorted as much as reasonable for quick routing once it clears customs in whatever FTZ the carrier is using).
Very few things outside of accessories or older SKUs with much less/more sporadic demand sit in US warehouses...it's just not cost effective with how expensive, even in the middle of nowhere, US warehouse space, staffing, and the like is (not saying that is good or bad, have zero interest in getting into that inevitably political discussion), it's cheaper to use UPS' logistics offerings with offer way more a just in time model than have to bulk import items, then have US labor do the 3PL work state side, because it's not just more expensive, it's double handling something when you don't need to when the end of the production line can quite literally feed the logistics operation.
That is part of the reasoning behind pre-orders, because the first waves don't have to go through double handling.
When you have the demand to be at-or-near production output (or behind it in this case), it would be leaving money on the table to do anything else.
Wave 1 & Wave 2 of pre orders are the only thing in the states now, and they don't even appear to have cleared customs, yet, still sitting in the KSDF FTZ.