Where Does Weather Forecast Come From

Several other older threads asking this but no answers but they are older threads.

To me a weather forecast is only of any use if you know the source and can assess its reliability. Some weather forecast sources are nothing more that marketing/ad presentation, others genuine and higher reliability.

So important that Garmin identifies the source of it's forecasts presented.

I can find no info  Does anybody have any idea where the forecasts come from?

Weather forecasts are part of outdoor activity safety  Would you go mountaineering without knowing the reliability of your ropes, or diving without knowing the standard of air put in your tanks, etc  Weather forecasts are similar  Just "don't rely on it" would make it nothing more yhan a dangerous toy  

Thanks

Ian