They don't extract a 3D mesh from the signal -- this doesn't literally "see through walls". They use the 3D mesh to model how they think they signal would be m

They don't extract a 3D mesh from the signal -- this doesn't literally "see through walls". They use the 3D mesh to model how they think they signal would be modified by the person behind the wall, then they compare the received signal with their model to see if it's a match.