Can someone explain how an organization like the NSA manages to keep records on every digital transaction (supposedly)? This seems like an impossible physics pr

Can someone explain how an organization like the NSA manages to keep records on every digital transaction (supposedly)? This seems like an impossible physics problem because the amount of data seems unfathomable even for an organization like the US govt. Pre-Snowden you could say that thinking was a conspiracy theory. Now we know that not only is it not a conspiracy, its probably much worse. How about Youtube? They amount of video uploaded daily is increasing yet they manage to store everything essentially on demand going back to the first Youtube video. At the end of the day for Youtube the data must be fetched from a hard drive somewhere...right? Are they buying thousands of HDDs daily?