[danbruc]

We still listened in on signals from the entire visible universe, but signals from further away are also from further back in time, so any aliens there would have had to develop earlier. But this also has an advantage, we could still hear from aliens that went extinct and only their signals are still propagating through the universe.

So the better way of thinking about this is a 75 year thick slice through spacetime containing the edge of the visible universe shortly after the big bang at one end and the solar system over the last 75 years at the other end. The volume between the past light cones of Earth now and 75 years ago.