Classical relativity has been known to us for centuries, ever since the bullock cart was invented by shepherds, the motion of moving bodies relative to each other was a known thing.
Let us think of an example of two moving trains close to each other, on two parallel tracks. Let us name them, train A, and train B. If train A is moving at 100miles/h & B is moving at 120miles/h starting at the same time from one station and moving in the same direction to reach another station, for an observer on the platform they would be moving at the above speed’s, but for an observer inside the train A, train B would be moving at 20miles/h which is the velocity of train B – train A (120-100=20).
Think of another example with a moving cargo truck in the constant velocity of 80miles/h, a baseball team, and a coach with a speed gun on the vehicle. When the truck is in motion and when the ball is been thrown by a team member who always throws it at a speed of 100miles/h on the moving truck and the coach measures it on the truck, his speed gun would read 100miles/h, but if he was off the truck and measured it from a distance as a stationary observer while the truck passes him, his speed gun would read 180miles/h, which is the addition of the speed of the truck plus the ball, if the ball was moving in the same direction as the truck. Therefore, we can conclude that the motion of the ball relative to the truck and relative to a stationary point on earth are two different things. And the other important thing that we learn from this example would be that being at a constant velocity and being at rest is the same.
(The System ISBN 9781631021008)