Transistors are more "3D" now, that's why a 10nm process gives you something like the same density as what a 2D planar transistor with 10nm minimum gate length

Transistors are more "3D" now, that's why a 10nm process gives you something like the same density as what a 2D planar transistor with 10nm minimum gate length would. See "FinFET" or "GAA" transistors If you're talking about stacking entire chips, yes that's also used lots of places. The challenge there is getting rid of the heat from those chips. If you stack lots of chips it gets really hard to get heat out of the ones in the middle.