Depend on whether it’s a FSBO or agent represented. If the agent is representing the owner and agent put it on Zillow, then agent put his/her contact info. If it’s FSBO, then owner put their contact info. It’s really up to the person putting in the information. I don’t think Zillow knows who it is. I’m sure there’s a question in there, asking whether you’re the owner or agent, but I don’t think Zillow checks.
If you used Zillow before, then you’ll notice the new map now have yellow flag for recently sold homes, red flags for homes that is up for sale (agent or owner put it up on zillow), and blue flag = “make me move” house. You can use the map to pan around to the area you want and you’ll see the different flags. Quite cool actually. They also have option where agent/owner can put up pictures as well.