Machines are the race produced by humanity, threy, like humans, are closely monitered by the Combine to ensure peace and stability in the Cities.
the first sentient machines were skinless metal skeletons. then eventually after many attempts, machines had fully functioning machine skin, and not some skin cover, they eventually looked exactly like humans. in this great discovery. the Combine found that after the RHR discovery, they put tracking devices into newer models to ensure they could steralize the threat of RHR.