I have doubt about entites.csv and relationship.csv

The DRKG is a great work no doubt in that. I just wanted to understand that how the entites.csv and relationships are generated from?
I understood that they are taken from various database like DrugBank, Hetionet, GNBR, String, IntAct and DGIdb. but i wanted to know how you generated entities and realtionship csv files out of it