The radiation forecast data is based on 1-2km resolution satellite imagery for near term forecasting (nowcasts), further out is based on numeric weather models and some ML. This comes directly from what resolutions we can get from the weather satellites.
Currently we use:
GOES16 for North + South America
GOES 15 for Hawaii, some of Alaska (this will be replaced by up coming GOES17 due early next year)
Multiple Meteosat satellites for Africa, Europe and West Asia
Himawari-8 for East Asia, Australia and New Zealand
The coverage map below is still accurate until GOES17 comes online.
Hope that helps!