All of these kinds of alerts (amber, tornado, flood, fire, and presidential test) that I've received on the west coast of the USA have been based on very coarse

All of these kinds of alerts (amber, tornado, flood, fire, and presidential test) that I've received on the west coast of the USA have been based on very coarse regions e.g. entire cities, counties, or districts. Whether that is cell tower or not I can't tell, but I've always assumed it was.