In this part of the course, we assume conductors to be perfect conductors. This is a good approximation for many real-world materials, especially metals such as copper, silver, aluminum, iron, etc. There is no electric field inside a conductor. If you try to make a field inside, the charges in the conductor move around in such a way as to cancel your attempt.
Charges can exist only on the surface of a conductor.
The inside is neutral, i.e., there are equal numbers of neutrons and protons everywhere. Otherwise, there would be electric fields inside. All points of a conductor are at the same potential.