As I understand it, it's a combination of things: aircraft reports, atmospheric motion vectors (e.g. a cloud doesn't have any propulsion, so if a cloud moves 30 km in an hour, you have learned something about the wind), Doppler wind lidar, and satellite measurements.

The numerical predictions of weather models often have many vertical components as well, so solving it for ground level also requires extending the forecast to the air, depending on the model.

To predict anything on the ground, you have to predict air temperature, humidity, wind speed and direction etc. for the entire troposphere. The output of the models in altitude are as relevant to the meteorologists as the output on the ground for manual expert analysis.