A sibling comment mentions raster scanning: building up an image one pixel at a time. That is largely true for a single telescope (though it is possible to have

A sibling comment mentions raster scanning: building up an image one pixel at a time. That is largely true for a single telescope (though it is possible to have an array of receivers taking multiple pixels at the same time). However, VLBI is an array of telescopes. Using interferometry - measuring the interference pattern of the signals of each pair of telescopes - we get simultaneous data on a variety of angular scales and orientations. As the Earth turns, the sky appears to turn, and more points in the UV plane can be measured. Once you have enough data, you can essentially get an image by doing a Fourier transform of the data.