How was the bamboo made strong enough to serve as pipes for the organ?

I assume you are referring to the Las Piñas Bamboo Organ. What was done to make the bamboo strong enough? Nothing. Organ pipes don't have to be very strong; just strong enough to stand upright and bear their own weight. The fact that they are producing a tone puts no extra stress on them. Other than this organ, bamboo has not been used for organ pipes except for a few small, novelty organs.