Personal understanding of the ending:
1. The eldest son is an alien, he doesn't know it himself. In fact, all the kidnapped people were originally children of aliens and were put on the earth to do experiments. So they all have some weird problems, which are caused by not adapting to the earth, and then they adapt after a period of time. After adapting, the aliens took them away to do research to see if the aliens themselves could survive on the earth. The people who are put back are to continue doing long-term experiments, just like many scientists do when they do scientific research on animals. I came to this conclusion mainly based on the last few pictures. Two of them were aliens who took away grown-up children, and the last one is the key. Without those black lines, they put down a baby. How to do it, the aliens with supernatural powers must be no problem. And I think the previous information about the child's illness is a foreshadowing.
2. In the end, the intercom made a sound, it should be that the eldest son has been put back, and he still doesn't know where he is. It may also pave the way for the second part.
Personal opinion. . .
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