In the twin paradox, one twin remains on Earth while the other travels away from Earth and returns. The traveling twin experiences a change in velocity, while the Earth-bound twin remains in a relatively constant frame of reference.
According to the theory of relativity, there is no preferred or absolute frame of reference. However, in the twin paradox scenario, we can distinguish between the two twins based on their respective accelerations and experiences of changing frames of reference.
The traveling twin is the one who accelerates and changes direction, which implies that they are the one leaving and returning to the initial frame of reference. They experience the effects of acceleration, such as time dilation and length contraction. This acceleration breaks the symmetry between the two twins, and the traveling twin will be the one who is younger upon their reunion.
On the other hand, the Earth-bound twin remains in an inertial frame of reference throughout the entire experiment. They do not experience the effects of acceleration in the same way as the traveling twin does.
So, in summary, the twin who undergoes acceleration and changes direction is the one in the non-inertial frame of reference, while the twin who remains in a relatively constant frame of reference is in the inertial frame.