Genesis cars are made in Ulsan, South Korea, the home country for the automaker. But they are designed in Rüsselsheim, Germany; Namyang, South Korea; and Irvine, California.
While you’re absolutely right that Hyundai owns Genesis, the two brands are completely separate. As such, they have different manufacturing plants. Genesis cars are luxury vehicles with higher quality materials than Hyundais, so they require a different manufacturing environment.