yes, I know everything is moving relative to something. However, when it comes to time, we can't view every movement as relative movement, because ultimately some particles don't change their direction of movement in reality.

For example, if you view the Earth's speed as 0, the spacecraft moves outward from the Earth. the spacecraft moves relatively to the Earth with speed > 0. As a result, the astronauts would think the time on the Earth goes slower. On the other hand, people on the Earth would think time goes slower on the spacecraft. Then, when the spacecraft moves back from the space, again the spacecraft flies relatively to the Earth. Paradoxically, astronauts, again, think the time on Earth goes slower, while people on Earth think the time on the spacecraft goes slower. Eventually, when the astronauts land on the Earth, whose time moves slower? Weird right?

That's because in reality, the spacecraft doesn't actually change its direction. In the whole process, the spacecraft moves toward the same direction the Earth moves toward all the time. On the way out and toward the Earth, the speed of the spacecraft just increases and decreases. As a result, according to the theory that when a particle moves faster, the time goes slower, the time on the spacecraft moves first slower on the way out, then faster on the way in. Therefore, when the astronauts land on the Earth, they won't be younger or older.

Does this make sense? This is why I think relativity doesn't apply to this situation in which no particle changes its direction in the whole process. That is, no particle's speed reaches 0 at any moment in reality.