You can use an iPad charger if you want, the power brick and the device communicate, and will only charge at the highest speed that the device allows. Using the iPad charger basically is just a higher wattage charger, so it will charge faster if you use a cable and device that support it.

The cable from the box is usb-c to lightning and is capable of power delivery (PD) of up to 30 watts. The usb-a to lightning cable can only go up to 12 watts. You have to use a corresponding power brick that supports the cables.