one can buy a variety of super super cheap very small boards or pigtail adapters with dc barrel jacks, in the usb-pd standard voltages: 5v, 9v, 12v, 15v, 20v.

one can buy a variety of super super cheap very small boards or pigtail adapters with dc barrel jacks, in the usb-pd standard voltages: 5v, 9v, 12v, 15v, 20v. You asked how does that work. Well, search for "usb-pd negotiation" and find out. Typically a source enumerates what it can do (lists some "power data objects", pdos) and a device files a request based on one, but I believe devices can & do ask for arbitrary things they want, especially if they dont find what they hope for, & them latter fall back to slower often 5v backup voltages if they can (or not). Thats the two sentance summary but theres a ton of info & its easy to learn more & you can download the spec & find out yourself if searching around still leaves you eith questions.