Multilayer PCBs consist of three types of layers: 1. the core on which the copper is attached, 2. the prepreg layers which insulate the different copper layers from each other, 3. the copper itself. So the multi layers on a pcb usually refers to the core layer,prepreg and copper layer.

For a multilayer PCB, the additional layers are stacked inside the plastic. The vias cross all the layers or just some of them, like buried vias.

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