What factor determines the cycle of light and dark observed on the visible near side of the Moon, known as lunar phases?

The changing angles of the Sun, Earth, and Moon

The phenomenon known as lunar phases is entirely dependent on the constantly shifting geometric configuration between the Sun, the Earth, and the Moon in their orbits. Lunar phases describe the cycle representing the varying proportions of the sunlit portion of the Moon that is visible from Earth at different times. It is not caused by a static shadow cast by the Earth (that only causes an eclipse), nor is it related to the Moon's rotation speed, which is fixed due to tidal locking. Instead, as the Moon orbits, the angle from which we view its illuminated surface changes, creating crescents, half-moons, gibbous phases, and the Full Moon.