These chargers/power supplies show multiple voltages such as 5v or 9v. How do they supply both? Different pins, or do the power sinks/devices to be charged negotiate a voltage or ??

These power adapters are smart devices, and can produce any ONE of the requested Voltages at a time.

They produce that Voltage on request from a smart device using USB Power Delivery Protocol. There are a few more pins on the interface, representing a simple slow USB bus, and they send messages back and forth to negotiate from what the power adapter can produce to what the smart device wants to receive over it ONE power line.

Power adapter connected is NOT necessarily charging, unless they have agreed to send and revive charging power at the required level at that time. For a MacBook Pro, that is typically the 20 Volt level to do a deep change that produces a charge cycle.

Your Mac will never overcharge.

Only the small 5V power is available without additional negotiation between the smart device and the smart power adapter.

So you can definitely run and possibly charge your phone at 5 Volts off the raw power outlet, but higher Voltages require more of the USB Power delivery protocol be implemented.

It sounds backwards, but you may need to get AC power from your car, and use the MacBook Pro power adapter to charge the MacBook Pro.