Many, many things. The setting I am working on is largely based on Spanish Era Philippines. It was a bit difficult because aside from the political climate and the succeeding revolt, nothing else gets told in schools. Upon researching, I learned about
The Tabacco factories, an Orphanage in the middle of the island in a river, the purpose of the layers of clothing, and the reason why big buildings do not have sharp edges.
Also, I looked into judicial systems in Victlrian era and also a bit of the Orphan train.
I learned quite a lot actually.