I have never used them myself, but I often hear about PCBs with several layers of copper. My question is how would you solder components onto the layers that are 'inside' the board?
connections are made via via's (no pun intended). The masking around the other layers ase such that there is no connection made.
It is common practice if you are making a multi-layer board to have aGND plane (for noise imunity and easier power tapping), power planes (for easier power) and then as many planes as required to meet board area
treat the horizontal lines as a plane. the square brackets are layer via connections As you can see with this setup the top layer, 4th and bottom are plated through and short them all together