The so-called copper coating is to use the unused space on the PCB as the reference surface, and then fill it with solid copper. These copper areas are also called copper filling.
Eight Principles of PCB Design
Build up Multilayer PCB, BUM refers to the application of insulating dielectric on the insulating substrate, or the traditional double-panel or multi-layer board, and then electroless copper plating and electroplating copper to form wires and connecting holes. A multilayer printed board requiring a number of layers.