The American sponsor? Could be anyone she's met there
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It could be a US “sponsor” who gave her a room in his house for free: an old grandfather/friend of her mom's who used to come with gifts to Ukraine before the war or another US family who wanted to help her and agreed to be her “sponsor” under the refugee program from Ukraine or her new American girlfriends.
If she were visiting places with Danila, he would be videotaping her all the time, but we see that most of the videos are of her filming herself. The conclusion is that she came with someone to the place in the video, she was filmed a few times and then she walked there by herself separately from the people she came with.
07.06.2024:
" Asya. We miss you very much. At the end of July, we're going to Berlin for an interview. I hope everything will go well and we will get a visa and fly to visit, and Alice and Taya will see their sister. "
In a month her mom and her sisters should get a visa to the USA and fly to visit Asya. The cost of renting an apartment in the city where Asya lives is very expensive, so maybe they will live in the house of Asya's “sponsor”, maybe her mom will show on YouTube / Instagram story video the sponsor's house and the sponsor's face.