"The pectoralis minor helps stabilize the rib cage into the arched position when the scapulae are anchored by the traps and rhomboids"
How scapulae are anchored by traps and rhomboids? I didn't get this line
The trapezius and rhomboids are adductors (or retractors) of the scapulae, meaning they pull them backwards. "Anchored" in this context just means "held in place".
So the traps and rhomboids are pulling the shoulder blades back and holding them in that position. A common cue for this is to tell the lifter to try to use their shoulder blades to pinch and hold the fabric of the bench.