That makes a difference, if at all, in only one direction. Amazon at least used to let you specify "I own it" for books that you own but didn't buy through them, which greatly improves their book reccomendations.

Amazon will also show you "I'm reccomending X because you bought Y", and let you say "don't use Y to reccomend me things". Youtube doesn't.

And the three dots don't appear for the one reccomended video that lurks among the related videos on the right of a video you're watching.