Gen one Synths are not people in any sense. Those are the creepy ones with no skin.
Gen two synths and similar models (Such as Valentine) possess human emotion and memories, but machine parts.
Early gen threes were mostly human, but had some synth components and some mechanical skeletons (See Broken Mask).
Late gen threes include Coursers, Diamond City's Mayor , and all that the Sole Survivor must rescure if working with the Railroad. They are indistinguishable from humans apart from a component in their brains.