Not sure where you're going with the "power supply work in reverse order". A USB-C connector is simply a connection type; the connector itself is bidirectional and does not of itself determine the direction that power flows.
USB-C Power Adapters supply power to the device to which it is connected. USB-C devices, such as your iPhone, will supply limited power from their internal battery to connected accessories, such as the USB-C Digital AV Adapter. However, this Adapter allows power to flow in the opposite direction, to charge the iPhone, when connected to a suitable USB-C power source.
In most countries/regions, Apple no longer includes a Power Adapter in the box with its devices. In those regions where the Power Adapter is now an additional cost accessory, the device cost is reportedly reduced to reflect this deletion from the box.
As outlined by MrHoffman, most models of iPhone will optimally charge when used with a 30W USB-C Power Adapter that supports USB PD (Power Delivery). These are sufficiently powerful to optimally fast-charge your iPhone - and can provide adequate power for both the iPhone and a connected Adapter that supports pass-through power. Lower power options can also be used, but will reduce the effective charging rate.