When you are sexually aroused, messages from your brain travel down nerves to your penis. Chemicals called neurotransmitters are then released from the ends of the nerves in the penis. Stimulation of the penis can also cause local nerve endings to release neurotransmitter chemicals.
The neurotransmitters which are released in the penis cause another chemical to be made, called cyclic guanosine monophosphate (cGMP). This chemical causes the arteries in the penis to widen (dilate).
This allows extra blood to flood into the penis. The rapid inflow of blood causes the penis to swell into an erection. The swollen inner part of the penis also presses on the veins nearer to the skin surface of the penis. These veins normally drain the penis of blood. So, the flow of blood out of the penis is also restricted, which enhances the erection.
Once you stop having sex, the level of cGMP falls, the blood flow to the penis returns to normal, and the penis gradually returns to the non-erect state.
Blog source: https://www.regentstreetclinic.co.uk/erectile-dysfunction/