I was watching this Ted Ed video on Youtube about the twin paradox and found the explanation with the spacetime graph a bit confusing. At 3:00 in the video, they show a graph and explain how bursts are sent by each twin to measure how much time has passed for the other twin.
My understanding is that at least during the first leg of the journey (until the traveling twin turns back), both observers must feel like time slows down by the same amount for the other twin, since their relative speeds are the same. However, the video seems to indicate that Terra feels that one year has passed for Stella when 2 have passed on Earth. But Stella sees that one year has passed on earth when 4 have passed on the spaceship. This means Terra feels Terra's time going twice as slow, but Stella sees Terra's time going 4 times slower.
In the diagram above (from the video), the first half of the journey doesn't seem symmetric. Is this right or is the explanation in the video incorrect?
ps: I also found some diagrams online that show this symmetry during the first half of the journey like the following and I'm asking if the video got it wrong and should have drawn it like this instead.
