They deposit half the layers, etch holes through them and form vertical strings of memory cells, then repeat all that to build a second deck of layers on top. T

They deposit half the layers, etch holes through them and form vertical strings of memory cells, then repeat all that to build a second deck of layers on top. The interface between decks is a potential source of problems, but is more manageable than trying to drastically increase the aspect ratio of the holes that need to be etched through the stack. Etching through lots of layers at once is why 3D NAND flash memory is actually economical, compared to basically every other kind 3D chips where layers have to be built one at a time.