The whole apparatus is fixed on Earth. As Earth rotates, the difference in signal is mapped out over a strip of sky. This strip has a width of 5 degrees, and goes completely round the sky once a day.
This mapping has been carried out day after day, for years. Maps of each day where added together, in order to increase the sensitivity and detect smaller and smaller fluctuations in the signal.
The final result is a map of the circular strip on the sky in the microwave range.