All 3 models (6-port, 12-port, and 18-port) can be bus-powered by the USB-C device (iPad Pro, MacBook, laptop, etc.) and do not require external power to work.
They can also be powered by an external USB-C PD power supply. In the case of the 18-port, there is also an option to use an optional DC power supply. Once externally powered, the hubs will provide power pass-through charging to the USB-C device.