I find the Korean call center paper pretty interesting [1]. In early March, this building had 97 infections: 89 on one half of one floor, 5 on the other half of the floor, and 3 on other floors. The two halves of the floor were separated by doors but shared the elevators, bathrooms, etc. It seems clear from this that almost all of the infections were from being in a shared space with someone infected. (There's a nice diagram of the infections [2].)

The other thing I got from the article is that South Korea is on a whole different level of testing than the US. When infection was detected, they tested everyone who was in the building or had visited: 1143 people tested with results in 12-24 hours. Everyone positive was isolated and everyone negative was quarantined and re-tested for 14 days. They also texted 16,628 people who had been near the building for more than 5 minutes (from cell data), telling them to avoid contact with others and get tested. This level of testing and tracing got South Korea (51 million people) to the point that it is now averaging just 10 new cases per day.

[1] https://wwwnc.cdc.gov/eid/article/26/8/20-1274_article [2] https://wwwnc.cdc.gov/eid/article/26/8/20-1274-f2