i think first they'll use whatever points of contact exist between the undocumented immigrants and the state, for example putting out a notice for the immigrants to go and report and get deported voluntarily. then they'll probably have raids using the last known residence of the undocumented people from employment, telecom, social media records gathered automatically and passed on by big corporations to sheriffs, ice etc using tech developed by corps like palantir if i had to guess
United States