Symmetrical editing

In this video

@0:30 how does she deselect vertices at mirror, and later on edit mostly using that function.

I’m trying to make a fishnet shirt but the vertex loops/seams are irregular what direction should I go in simplifying the geometry to something like a grid but in the shape of the torso. From there I think I could bevel and scale the grid.

I don’t see any deselection at :30.

Oh sorry I meant at the beginning 0:03

After typing ‘C’ to bring up C select, use left mouse button to select, middle mouse button press to deselect.