Are US iphone 12 dual sim?

Is it because of the inserter/SIM holder provided with the device or the hardware inside the phone itself doesn't support it ? and is it the same even for fully unlocked phones not tied to any carrier ?

No, they will have one eSIM, one Nano sim slot.

Why not used dual sim? if i purchase in t-mobile my iphone 12 pro max and in amazon purchase Tray Holder Slot Adapter for dual sim, my 2 physical SIM card working, my iphone will recognize it?