(Beginner question) The different ways a transistor can be connected to power source
For conventional current flow going from (+) to (-)

NPN transistor


inject current into base causes small current to flow from base to emitter
when the base is accepting current it will be about a diode drop more positive than the emitter
with the base current there can be current flowing from collector to emitter which can be larger than the base current
collector will normally be setup to be more positive than the emitter


PNP transistor


extract current from base causes small current to flow from emitter to base
when taking current from the base it will be about a diode drop more negative than the emitter
with the base current there can be current flowing from emitter to the collector which can be larger than the base current
emitter will normally be setup to be more positive than the collector


Keeping the above in mind, which are the proper biasing and current directions for operation of the transistor, you get to select just what represents the power supply to a given circuit. There are many ways to place circuitry around a transistor that abide by the above "rules" and provide a functional circuit.