The Moon does not look the same every night. Some nights, it is a thin curve. On other nights, it is a bright circle.
The Moon does not make its own light. Moonlight is sunlight reflected from its surface. The Sun always lights half of the Moon.
As the Moon orbits Earth, we see different amounts of its bright half. This creates the Moon’s phases. The Moon itself does not change shape. One full cycle takes about 29.5 days.