How do I make the average of two or more scenes in SNAP or QGIS?
You need to have them in a layer stack first.
For SLC radar data this requires coregistration (Radar > Coregistration > Coregistration)
For GRD radar data, Terrain Correction and Stacking is often sufficient (Radar > Coregistration > Stack Tools > Create Stack). If they spatially do not match, coregistration is the better option (at best before Terrain Correction).
Then you select Radar > Stack Tools > Stack Averaging