All tubes and transistors can be simplified by thinking of them as 3 pin devices:
Input (music), Output (Music), power.
Between the input and the output a device increases voltage, or current, or both.
Amps tend to have multiple stages, up front there’s a voltage amplification, at very low currents. The standard is about 20x from input to output, then at the end is a current stage, which may have no voltage amplification but can deliver the current the speakers need. That’s usually what a power tube or transistor does.
This is why tube amps will usually have at least 2 different types of tubes. The smaller one's doing voltage related work, the larger one's current.
While voltage goes up by around 20x, current must go up by say 40,000 : 8 or 5000:1 and this is very difficult to do with single stage amplifiers.