It's basically as simple as a buck converter that reduces the incoming voltage. Phone batteries need either 4.2V or 4.35V to charge fully, so whatever voltage

It's basically as simple as a buck converter that reduces the incoming voltage. Phone batteries need either 4.2V or 4.35V to charge fully, so whatever voltage comes in (5-20V typically) at whatever current, gets reduced down to the correct voltage and current to charge the battery.