I would like to understand how they aggregated the data for a specific location (pixel in that map?) and how they obtained temperatures - is it a temperature me

I would like to understand how they aggregated the data for a specific location (pixel in that map?) and how they obtained temperatures - is it a temperature measured by a satellite or measured by a weather station on ground; if it's on the ground how many stations they used and how often was the temperature sampled. They discuss very little about this and I think it's key to understand this data they present here.