Mechas are humanoids who were once human. These hybrid-humans have embraced The Singularity to augment their bodies with enhanced physical capabilities.
Mechas believe that their coexistence between man and the machine make them enlightened beings who have surpassed the human race.
In Episode 7 Sentients, Lithium tells Rachel that the Mechas were recruited to build Tier City and the Bio Domes. It is yet unclear how this agreement was brokered, and if the Mechas were put to work willingly.
Rachel believes that the Mechas are "prone to uncontrollable fits of violence." Lithium dismisses this as another lie propagated by The Preacher.