The fly over is a fact gathering terrain mapping to aid the CCP in their initial land invasion from Canada into the most unpopulated northern states.
The initial attack will include a power grid failure to large swaths of the US, which will cause chaos and divided resources while Chinese infantry take their first territories.
If General Milley didn't give them all of the info they need, then Fartwell, Schitt, and Feinstein probably already did.