The current is the flow of electrons in a circuit and the voltage is the energy that is carried by the electrons. The current is the source of electrons and the voltage is carried by them.
Because the bulbs in the parallel are separated because it has multiple pathways. so the current that goes to that pathway where the switch is closed, it focuses the current to that bulb. A series is a one way circuit so it uses all the voltage at once when the switch is closed, the current goes around the one way circuit, so if there’s more bulb it’ll use less voltage and will result less brightness.