All of the power-related pins are groped in one section on the board, close to the microUSB connector. 

VBUS – This is the power from the microUSB bus, 5-volts. If the Pico is not being powered by the microUSB connector then there will be no output here.
VSYS – This is the input voltage, which can range from 2 to 5-volts. The on-board voltage converter will change it to 3.3-volts for the Pico.
3V3 – This is a 3.3-volt output, from the Pico’s internal regulator. It can be used to power additional components, providing you keep the load under 300ma.