Phantom power is a small DC current sent through the mic cable.
In the case of a regular PA system it originates in the sound board and is transmitted through one wire of an XLR tipped cable. There is usually a button for turning the PP on or off on boards that have this capability.
In the case of a computer it runs to the usb port and then through the cable to the mic.
Some instrument amplifiers also have phantom power.
a transformer within the sound board, amp, or computer is the actual “source”