In projectile motion, the velocity of an object does change as it moves through its trajectory. However, the speed remains constant while the direction of the velocity changes continuously. This is because projectile motion consists of two independent components: horizontal motion (constant velocity) and vertical motion (constant acceleration due to gravity).
In the horizontal direction, there are no external forces acting on the projectile (such as air resistance), assuming an idealized scenario. Therefore, according to Newton's first law of motion, the projectile will continue to move with a constant velocity in the horizontal direction. This means that the speed of the projectile remains constant throughout its horizontal motion.
In the vertical direction, the projectile is subject to the force of gravity, which causes it to accelerate downward. The velocity in the vertical direction changes due to this acceleration, resulting in a changing vertical component of the projectile's overall velocity. The velocity in the vertical direction starts at zero when the object is at the peak of its trajectory, increases as it falls downward, and reaches maximum at the lowest point of its trajectory. However, the speed (magnitude of velocity) in the vertical direction is not constant due to the influence of gravity.
So, while the speed (magnitude of velocity) remains constant in the horizontal direction, the velocity does change in the vertical direction due to the acceleration of gravity. The combination of these two motions (horizontal and vertical) gives rise to the curved trajectory observed in projectile motion.