Does the red light synthesize collagen just where it hits or throughout the body?
Red light interacts with the body in a similar way to infrared heat.
The key difference between red and infrared is that red wavelengths of light are absorbed and used in the first 3-10 mm of skin tissue, whereas infrared heat can penetrate much deeper into the body and be transferred throughout the body with blood flow. So for best results, use red lights on bare skin. The majority of collagen will be synthesized right where the red light hits but there would be some overall bio-stimulation effect (like with FIR heat).
A 2014 study published in Photomedicine and Laser Surgery demonstrated both efficacy and safety for red light therapy in promoting anti-aging skin rejuvenation and intradermal collagen increase when compared against controls.