but how could a dev make an upload button for youtube on some other domain?

It's too complicated. We don't even think about it. What we want to do is give users of other social media a chance to monetize their content on Blurt. They don't even have to be active on this blockchain. All they have to do is create an account on Blurt and link their social media profiles to a cross-poster, which will detect (via rss, api) and publish links to their posts on hive, steem, wordpress, bitchute, rumble...