Click the small circled i in right bottom corner of your screen, or see About this data in the mobile app.
For example:

Capture d’écran 2022-01-19 à 10.05.27.png

Copernicus use Sentinel-5 satellite observation and altitude wind forecast from ECMWF model.

Note that the plume of SO2 is based on the total amount of this gas in a column from ground to the top of atmosphere. Reason why it is expressed in mg/m2, so it is not at ground level