Follow the idea
Redox reactions involve electron transfer or, more generally, changes in oxidation state. Oxidation and reduction always occur together. Tracking oxidation numbers is a practical way to see which species loses electron density and which gains it. A galvanic cell converts the free-energy change of a spontaneous redox reaction into electrical work by separating oxidation and reduction into half-cells. Electrons travel through the external circuit while ions move through the electrolyte to maintain charge balance. Electric potential is potential energy per unit charge. Potential differences tell us how much electric potential energy changes per unit charge between two points. Voltage is therefore not a substance flowing through a wire; it is a difference in electric potential.
Primary references
IUPAC Gold Book · NIST · BIPM