Google/YouTube will basically pay Sam x amount per view, as he has got monetisation switched on. I believe He'll get something for the number of subscribers he has also.
Google/YouTube recoup this payment by charging advertisers to display adverts.
As a side note, I don't think he is getting directly paid by hornby or others as he would have to mention this on his videos. He has been given things in the past and he makes it very clear when this happens.