I'm not an insider so my informations may not be totally accurate but I know a 'new' beauty youtuber/blogger and apparently, she sometimes receives products directly from the brand. In her posts, she's quite transparent and says she got the product from the brand for a test but well yes, I suppose a lot of sponsored posts are 'hidden' (just like with fashion bloggers). It seems there's also some creative/marketing agencies which contact bloggers/youtubers and send them products from random brands. I suppose these agencies are hired by the brands to get the social media coverage without needing to create a whole new department. Or maybe because it's still quite 'controversial' to send products for this kind of 'advertisement'. As I'm french and talking about french bloggers/youtubers, I think maybe it's not the same in the US or in any other countries. I know that hidden advertisement in France is legally prohibited so brands may need to be careful on how they manage this kind of stuff.