The second and third surgeries are really two stages of the same operation. Most of us have two functioning ventricles: the left ventricle pumps (oxygenated) blood to the body and the right ventricle pumps blue blood back the lungs to collect more oxygen. Since Brea has only one functioning ventricle, the goal of these two surgeries is to overcome the need for one of the pumps (the function of the right ventricle). To do this, they take the two veins that bring blue blood back to the heart: (i) the superior vena cava from the upper body and (ii) the inferior vena cava from the lower body; and connect these to the pulmonary artery. This means the blue blood bypasses the heart and goes directly into the lungs for more oxygen.
The two different veins provide the breaking point for the two surgeries. In the Glenn, they connected the superior vena cava to the pulmonary artery and in the Fontan, they hope to connect the inferior vena cava to the pulmonary artery. This means that at present, Brea is still relatively blue (her oxygen saturation levels are around 80%), but after the surgery they should run much closer to 100%.