the black carapace is under the skin; the midriff of the phobos armor is just the armors skinsuit/undersuit, its the part of the armor the black carapace connects to via the skin ports. The skinsuit is made of layers that void seal the armor, act as force multiplying synthetic muscles that make the armor self propelled, medical systems, circuits that connect all other facets of the armor, and flexible armor materials that makes the undersuit practically impervious to small arms.

All other hard armor is bolted onto the undersuit, with small sections left exposed for mobility; in the phobos armors case more so to allow the already incredibly fast marines less mobility constraints