The "hot" trackers try to measure full exposure of the song through different channels, buy aggregating data from radio airplay, sales, and streaming.
Radio exposure is measured by a subjective concept of "audience impressions" (quoting from Billboard's own site). Normally this would be evaluated
by using marketing measurement methods, i.e. inquiry of a sample audience, not the number of times the song was played (that won't tell you how many people actually listened to it). But it could be a complex method involving number a plays, yes, together with global audience ratings for each show, etc.
The different channels have different weights to (at least in theory) account for the different characteristics. I don't know about radio, but I know that regarding streaming, 1500 plays (yes, 15 hundred) count the same as the sale of 1 unit of a physical product (e.g. CD or Vynil).