The Twins paradox is well known and apparently well explained.
The explanation does suffer from one problem however. It uses a zero for one of its parts. Zeroes are nasty things. You can easily ‘hide’ things unintentionally by reducing it to unimportant by inclusion of a zero multiplier.
So let us considered a more balanced view of the problem.
Three people, not two.
One remains here on Earth as a reference.
One departs in the direction of Alpha Centauri.
One departs in exactly the opposite direction.
Both the moving parties are travelling at some relative velocity close to the speed of light.
Both turn round after some period of time (as observed by them) and return to Earth.
Now consider who is younger of the three?
The one on Earth? He observes that both the moving ‘twins’ are the same age. Same velocity magnitudes. Different signs but v2 removes that from consideration doesn’t it?.
The one who went towards Alpha Centauri? He observes that the other ‘twin’ is younger than the one on Earth as the relative velocities are stacked that way.
The one who went away from Alpha Centauri? He observes also that the other ‘twin’ is younger than the one on Earth as the relative velocities are again stacked that way.
So who is the oldest and how (or indeed why)? A problem to be sure!
A related problem. GPS. It is well understood that GPS require adjustments due to relativity. Precisely calculated and validated.
But when you stand back, the relative velocity when observed from any given point on the Earth’s surface only sums to zero over any given period (at integer multiples of orbit).
So where is the relative velocity component that we are adjusting for?