A Bipolar Junction Transistor (BJT) with a voltage divider across its terminals will build up an adjustable amount of voltage across it. Even as supply voltage changes.
Once the potentiometer is set, the transistor will conduct current better as the voltage across it tries to go up.
Whereas the transistor will conduct less current when the voltage across it tries to go down.
So the transistor actively pass current as needed to maintain a voltage across the collector and emitter.
Maximum current needs to be limited by some more resistance on the high side (closer to Vcc) of an NPN BJT.
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