The phases of the Moon depends on the position of the Moon relative to the Earth and the Sun. They occur because the Moon is spherical & lit by sunlight and that the Sun is very far away. Both imply that the Earth is spherical too.
The phases of the Moon according to observation cannot be explained in any flat-Earth model. Flat-Earthers had to invent various explanations to explain how the lunar phases can happen in the flat-Earth model. None can explain how a ball has the same phase as the Moon when seen from the same direction as we see the Moon.
We can try a simple observation: hold a ball —like a ping-pong ball — at arm’s length, and aim it at the Moon when it is visible during the day. We can see the ball shows practically the same phase as the Moon.
This phenomenon can only happen if the Sun is very far away, and both the Moon and the ping-pong ball are lit by the Sun.