Every computer component gets power supply from motherboard only. It is the responsibility of the motherboard to circulate the power supply to “each and every component” as soon the computer is booted. Usually keyboard, mouse, pen drive, CD / DVD drive, floppy drive, etc gets power supply directly from the motherboard.
If the “keyboard and mouse” are wireless, then motherboard transfer power supply only to the sensor that it connects it and keyboard and mouse works with the help of battery. The other devices like printer, scanner, modem, etc gets power supply directly from the power board or electrical board.