The only question that I am confused with is that, what if the streamer won huge using that lent bank roll? Will they give him additional payment?
From what I've seen from many YouTubers that I watch who is being sponsored into various websites AFAIK they get to keep less than 5% of the amount they won. But surely it depends on the contract between the site owner and the content creator.
We do not know how much they won from that site because the contract will not show to the public, and only the Youtubers and the site will know. Maybe the streamer will use the other account to playing gambling on that site, so the public will not know if they win more or lose more. But I think the site pays the streamer with enough money to promote their site with good content to convince the audience.