How is voltage exactly created in a citcuit?
You should start from a physics course. Learn what charge is, what is electric field, what is electric potential. 

Basically negatively charged particles are attracted to positively charged particles. So if you have them separately, they will flow one to another and release some energy in the way.

Circuits are just engineering of physics. Chemical reactions in battery separates atoms to ions (positive charge) and electrons (negative charge), while wires allow them to flow.