How to measure global temperature?
Measuring stations are used, but to fill in the gaps that are inherently there. Satellite observations of surface level temperatures are used to smooth out the data (and gather information about the oceans) to create a full look at the global temperature.
Then the observed temperatures are compared to averages from recorded history and hot and cold spots are determined. Then finally the temperature deviations are added and subtracted respectively from each other until all deviations have been accounted for and a determination of above or below average is determined.
ECMWF uses their Copernicus satellites for this type of research which is documented in this presentation. The data is available through their ERA5 database.