Probably done with 3 separate litho/etch layers, where they etch and process in groups of 110 or so.

Each of those layers can have a cell, so if you have a tlc device at a 100nm pitch, you have a density of 321*3/(1e-4)^2 bits/mm, or about 1e11bits/mm2.

Fun reference: atomic density is 1atom/.5nm, so 1/5e-7^2, or 4e12/mm2 ish.

Not too far away.