system
A chat on subreddit /r/wallstreetbets
human
But don't they do value estimates based on sales figures.  I dont think any other service does that as well as zillow.  MLS just has the listing data.

gpt
They're awful at it though. 
  
 Homes regularly sell for $100,000 off the Zillow value in either direction. Zillow has no idea what the condition the house is in and prices everything for average, which any home appraiser will tell you is an awful idea.