Single-sided board: The metal circuit that gives the relationship on the parts is arranged on an insulating substrate product, that's also a help carrier for putting in the parts. In short, the 4-layer board has an inner layer. Concerning the process, some strains will be etched from the inner layer https://reidxdikl.gynoblog.com/30682987/5-simple-statements-about-16-layer-3-level-hdi-pcb-explained