I worked at Redfin. MLS is an agreement between all of the brokerages to share their listings. As long as Redfin has a real estate office in the region then the

I worked at Redfin. MLS is an agreement between all of the brokerages to share their listings. As long as Redfin has a real estate office in the region then they will have access to the local MLS. On the other hand, Zillow requires agents to manually upload listings. This is why Zillow is nationwide and Redfin is limited to a (growing) list of cities.