I don't know if we ever will receive the precise details of this NSA operation, but I would still like to know:
1) How was the integrity of the shipping chain tainted? At which point NSA grabbed the devices and who allowed them to do this?
2) What does this "spyware" do, and does this mean a modified system firmware or something else?
Most of that is covered in Greenwald's book, and also in the NSA documents that have been released. The specific physical interception point is not described, but the modified firmware is. Once the router goes into service it "phones home" periodically and allows NSA to send monitoring instructions.