Synthetics or robots are a type of people. They're typically made of metal, as you would expect, but some take more esoteric forms such as programs or operating systems. Even those that have an obvious physical form are not always humanoid in shape. They range in their ability to vary from their initial programming.
Mechs are technically synthetics, though the majority are not conscious. Divines are usually considered their own unique lifeform distinct from other synthetic beings, but this is a question of popular perception, not a result of some inherent distinction between the two.
Stel Columnar is a society of synthetics.