It used to be its own species like humanity that populated it's solar system but then the flood infected them and the now version of the hive is a flood form but they still knew how to use their technology and with the knowledge from the gravemind they became capable of intersteller travel. But their flood side made them attack humanity and now they continue. This could explain why thrawls rush at you. They are like the infection form of the flood. And the knights are the hive version of the tanks and the acolytes are combat forms of the hive. And the wizards are a separate smaller gravemind which could explain how they can use the power of the void.