Hardware Reviews

On most of these hardware reviews you read or watch, do the reviewers buy these items or are they supplied by the manufacters? I'm assuming most are given to them. So do they keep them or return them?
Just wondering if it is the same thing I went through when I built models for a hobby. We were usually supplied them, in return for a review. Of course we kept ours since they ended up being constructed. Some of course were sent back for photos, box art, etc.
Just wondering...........