When a load is applied to a voltage source, it draws a certain amount of current.
A voltage source is the dual of a current source.
The current through an ideal voltage source is completely determined by the external circuit.
To take one example, consider the perfect voltage source.
Next, all voltage sources are converted to their equivalent current source representations.
That resistance reduction makes the combination a more ideal voltage source.
Do not forget that whatever voltage source you have is part of the closed circuit.
A high voltage source for the initial injection of particles.
Batteries can be approximately modeled as a voltage source in series with a resistance.
If the voltage sources do not match, at least one will have a huge current.