Thank you for this great question, Badalyanrazmik.
Attempt at an answer, though I am not a lawyer:
Officially, transcribed untranslated subtitles cannot be copyrighted in law, because copyright is for works of human creativity: transcribing is not creative. Of course, good captions for the Deaf and Hard of Hearing do entail a significant creativity: that's why e.g. TED always mentions the names of the transcriber and reviewer.
As to translated subtitles: the copyright belongs in part to the creator of the video and in part to the translator, as translations are considered derivative works.
Video authors keep the right to allow or forbid the creation of untranslated or translated subtitles. This is why Amara has a procedure for reporting copyright and trademark infringement.
Now as to the subtitles' creator in general Amara: when volunteers sign up for an Amara account, they should be aware that Amara works as a kind of Wikipedia for subtitling. I.e., several people can and often do work on the same set of subtitles, and the acknowledgment of each contributor's work is in the history of revisions of the subtitles.
Teams and Amara on Demand can have different terms than general Amara. If you want more detailed and authoritative information, feel free to create a ticket that will go to Amara staff.
Thank you for the detailed answer.
There may be creating aspects, like time syncing (esp for translated work) and editing for grammar, font choice and placement, etc.
Disclaimer: Not an expert on law, just an opinion.
Who has the copyright for the subtitles made on Amara? Do they belong to Amara, to subtitles' creator, or to the video owner? Or does no one own them and they are Public Domain?