I can tell you because it's in the MI5 propaganda Spooks.

There are supercomputers available with sorting algorithms. These computers can trace a phone using cell towers and download everything on the phone from pictures to contacts.

These supercomputers can trace every phone and only target phones which are flagged for observation. (But if they need to bring you to the police station they can use your data to ascertain probable suspicion.)

But in government data centers everyone's data gets stored until necessary. The faster your broadband works the faster they have already built the infrastructure needed to monitor you and everyone else.

Big government has big weapons that even people in IT don't know about; but you should assume they can blackmail you whenever they want.