When you are standing in a constant-moving elevator, what forces act on you besides gravity pulling you down and the floor pushing you up?
There are no other forces acting on you; it’s just those two. and because the elevator is moving at constant velocity, the net force on you has to be zero. You are coasting and that means that the force of gravity downward, which is also called your weight, is exactly balanced by the upward push from the floor. Those two forces sum to zero, so the net force on you is zero and you move at constant velocity.
We don’t necessarily know what that velocity is though. It could be that you’re moving upward at constant velocity, or moving downward at constant velocity, or even motionless. But as long as the two forces exactly balance one another, the net force on you is zero and you don’t accelerate.