I know how normal PCBs are etched, but every time I grab a magnifying glass and examine the edge of a multi-layer board I'm astounded by the precision that must be required during production. On top of that, some of them have buried vias and other such trickery, which must further complicate the process.
How are these boards made?
In a nutshell, to make a 4-layer board, 2 double sided boards and a separator are laminated together. You can extend this idea to 6, 8, n-layer boards.
Amazing isn't it?
A key to manufacturing multi-layer boards is the mechanical registration
system which ensures layers line up with sufficient accuracy.