Our ATC uses real-world METAR data collected from AviationWeather.gov. There are a few things you should know:
1) MSFS has no weather API, or any way for developers to get weather data directly from the simulator. For this reason, we ask all pilots to run real-world weather.
2) The exception to the above is if an aircraft is on the ground, then we are able to get the current wind information directly from the sim, for the current location of the aircraft. This means your takeoff clearance from ATC will match the exact conditions inside your sim. (Landing clearances, however, will use the wind information from the latest METAR data).
3) We refresh the METAR data for an airport / ATIS / AWOS / ASOS station once the data has been stale for one hour (This is to avoid overloading their servers with hundreds of thousands of requests every minute).
4) The time-stamp on the METAR is the date/time it was collected from AviationWeather.gov
5) If you choose NOT to run real world weather, then ATC may assign runways that are not appropriate for your wind, may give you incorrect altimeter settings, and the ATIS information may not match your conditions. As long as you are okay with these limiting factors, there's no reason you can't run custom weather.