Current flow takes place at the atomic level. It is dependent on the composition of the metal being used. Copper for example . Each atom is made with protons and neutrons that are bound in the center, just like the Sun. There are electrons that circle around the center, just like planets. The similarities end there. Atoms have a limit on the number of electrons that can be placed in each orbit around the core. Each orbit line cannot take electrons until the lower orbits are full. This orbit is caused by energy at the molecular scale. Because they are all negatively charged electrons, this orbit causes them to space between each other and repel one another. Each electron that is bound makes the previous electrons stronger. The last electron that fills this orbit is the one held the most. The copper element is’relaxed” when its outer orbit receives the last free electron. This happens because it’s in a balanced state. The next atom of copper, however, is “hungry” because it lacks a balanced charge and attempts to draw other electrons into the sphere. The electron leaps when one of the free electrons is offered to balance the other. Multiply this electromotive force and you get a cascading flow of water. More than billions and trillions of electrons jump from one atom to the next. The current is caused the the free electrons coming from the source (a chemical battery).
Think about this. The terminal that provides negatively charged electrons. The negative terminal! This means that current flows from the negative terminal to the positive. Years ago, we were led to believe that current flows from the negative terminal to the positive. However, on a molecular level, it was the opposite! Science didn’t know how current flowed or what it was. They guessed wrong. This is called ‘hole flow’ in electronics science because electrons move around currents and the holes appear to flow in opposite directions. The holes flow from negative to positive, that is how it works! It helped the older guys retain their understanding. Wikipedia has more information about this topic.