The output of a common emitter amplifier is a voltage signal that is proportional to the input voltage. All of the power for the output signal comes from the power supply VCC. The input signal Vin is just a control signal that determines how power from VCC comes out.
To be clear, a BJT transistor is a current controlled current source. The current through the collector is proportional to the current through the base. A transistor in a common emitter amplifier only outputs a voltage because of the attached resistors.
The base current is related to the base-emitter voltage by...
IB = Is * (e^(Vbe/n/Vt)-1) / (Beta + 1)
IE = (Beta + 1) * IB
IC = Beta * IB
IE is the emitter current
IC is the collector current
IB is the base current
Beta is the transistor current gain, typically on the order of 100
Is is the base emitter diode saturation current
Vbe is the base emitter voltage
n is an ideality factor, usually between 1 and 2
Vt is the thermal voltage, 26mV at 25C
e is 2.71828...
When applying small AC voltage signals to the base, around some DC bias point, we can form a linear approximation treating the base-emitter junction as a small resistor having resistance...
re = (dIC/dVBE) / ICQ = Vt / ICQ
IB = Vin / Beta / Re
IC = Beta * IB = Vin / re
re is the base-emitter resistance used for the linear approximation at IC=ICQ
ICQ is the collector current at the bias point.
The bias point is determined by the resistors attached to the base, and possibly to the emitter.
By adding a resistor RC to the collector, the voltage across RC becomes...
Vout = RC * IC
Vout = RC / re * Vin
The gain of the amplifier is RC/re. That's how the basic common emitter amplifier works.
There are alternate technologies to transistors. One of the most widely used ones in recent history was vacuum tubes.
One could even use a rotary potentiometer controlled by an electric motor to make an amplifier, but the practical frequency ranges would be pretty small. A typical transistor can pass signals at millions of cycles per second. Good luck designing a motor or potentiometer that can change directions that fast.