Not true in all cases...
At least for the Boeing 747 and 777 (and maybe others), the center wing box is assembled as a single unit, as are each of the external wing sections. The wings are then bolted onto the center box.
The last airplane I saw that had the wing built as a single unit was the A-4 Skyhawk. I'm sure there are others, but I haven't seen them disassembled...