The Wikipedia[0] has one interesting source[1] but what they show there isn't an average.
My guess would be that somehow satellites can measure the surface temperature and use that to make an average?
I would be very happy if someone could some point me to some explanations about this process.
[0] - https://en.wikipedia.org/wiki/Global_surface_temperature
[1] - https://www.ncei.noaa.gov/access/monitoring/monthly-report/g...