It is to “stack ”inner layers,prepreg (which is for insulation purpose) and copper foil of out layers (top and bottom) together.

It come out by using a combination of specific temperature(heat and cooling), pressure for a specific time to allow the resin within the pre-preg to flow and bond the layers together to form a solid multilayer panel.

Normally,there are many times of lamination process for some multi-stacked via and blind holes.