How are the three modes of the MacBook Pro usb-c charger being used?
The charging mode used depends on what is being charged. When you plug-in the charger to a device, the charger and the device will communicate with each other and determine the correct charging mode.

For non USB PD (Power Delivery) devices, it will default to the old USB standard, which is the ~5 volts.

For USB PD (Power Delivery) compatible devices the charger will support either 5, 9 or 20 volts according to what the device wants. The USB PD standard (rev. 3) defines four standard power levels that a charger can support: 5V, 9V, 15V and 20V.

Further explanation on Wikipedia:

USB Power Delivery