Although they are understandably hard to see in enough detail in the images, it appears at least two, possibly three beetle species are present. As such, their food sources may vary. They do all look like beetles, and would all be adults, but the variation in body shapes suggests they are separate species. One has a definite "pinched" area between thorax and abdomen, and its elytra (the covers for the wings on the abdomen) are fairly long and narrow. Another has a similar narrowing at this "waist" but is a shorter beetle overall, and its elytra are proportionately broader and the tips blunter. Others seem to have no "waist," in that their thorax abuts the abdomen with no discernable break or gap. Your suspicions about the various household beetle pests - Sawtoothed Grain Beetle, Powderpost Beetles, and others (here's another info. page) - are headed in the right direction.

Potential sources of food for the beetles are numerous. Across all the possible species, look at any of the following items for contamination: human food (dry grains, dried fruits, spices, and seeds), dry pet food or wild bird food, dry medications (non-gel pills, powders), tobacco in any form (cigarette, cigar, etc.), leather, wool, potpourri or dried flowers, rodent bait (in case the builder left some anywhere), and possibly the cardboard of the boxes themselves. If you have any wooden or bamboo decorative objects, it wouldn't hurt to check them as well. It's not likely they are in any wooden structural parts of the house, given how new it is, but if you see exposed, unfinished wood anywhere, that too is worth inspecting.

We agree that the box storage in the warehouse may have been a point of origin. It's hard to say if the bed frame is suspect; on the one hand, bored holes (only the size of the diameter of the beetle itself, so quite small) and sawdust should be detectable, but insect attack is less likely if the wood is coated with a finish; on the other hand, since many shipped products have been stuck in international transit for much longer than normal due to port backlogs, perhaps cross-contamination of hitchhiker insects is happening with greater frequency.

Other than inspecting objects and stored goods, all you can do is collect or vacuum-up the beetles as they appear. As their potential sources of food are removed (such as the cardboard) or used-up by the beetles themselves, they should finally disappear. If the infestation worsens or persists once moving boxes are unpacked and no items seem to have a beetle presence, then perhaps a pest control professional can identify them via a microscope in their offices. Some county-level Extension offices may be able to look at samples, but not all are open at this time to walk-ins. Periodic Master Gardener "plant clinics" occur in counties which have resumed such in-person activities, but they likely don't have microscopes in their possession, though individual MGs may have a loupe or other handheld magnifier with them. You could inquire with the lead Master Gardner in your area to see if this is something they think could be IDed at such a clinic, and when/where those clinics will be taking place. You can try a neighboring county too if that's more practical.

Miri