Possibly using spies and black bag jobs. Update: Further Snowden documents confirmed NSA used spies in Germany & Korea. The undoubtedly excellent engineers at Deutsche Telekom couldn't find how the NSA was tapping them. "Reporters for Der Spiegel, working in collaboration with The Intercept, contacted Deutsche Telekom and NetCologne several weeks ago in order to give them an opportunity to look into the alleged security breaches themselves. The security departments of both firms say they launched intensive investigations, but failed to find any suspicious equipment or data streams leaving the network." Those NSA guys have to be really good to fool the Germans.
We all "knew" that surveillance was everywhere, but continuing revelations from Laura Poitras & Edward Snowden remain startling. The latest report suggests heavy use of undercover agents and physical intrusion. That's what spies do, after all. England's GCHQ is deeply involved, along with the Aussies and the Canadians.