🚀 go-pugleaf

I believe it's done by calculation, at least for local AM/FM stations.
Given the transmitter power and the antenna characteristics you can
calculate zones of signal strength about the station.  You can correct
the calculations by making field strength measurements in the field (no
pun intended).  You can draw these on a map - for AM you have to have
separate day and night calculations if the station operates at night.
I remember seeing in a small-town newspaper, when the local AM station
was going for a power increase, a map of "before" and "after" coverage
within which the signal is strong enough to be easy to listen to.
>From that point you can count the houses on the map and estimate the
number of people per household, etc.

That takes care of the simple case out in the boondocks where there is
only one local station.  For urban areas I guess they have to estimate
the percent of population listening to each available stations by some
other process.