Question

what creates a voltage source?

Answer 1

the current in a circuit.

Answer 2

Your question is extremely broad. At its simplest, a voltage source is nothing more than an imbalance of electrons between 2 terminals.

This can be achieved mechanically through the use of generators, alternators or rubbing amber rods with wool cloth. :-)

Or it can be achieved chemically as is done with batteries.

Current doesn't exist until a conductor (e.g.: a "circuit", AKA a "load") is placed across the terminals of the voltage source.

Answer 3

instability of electrons