Where all the planes exactly the same?
All the planes were different, it does not matter if they came from the initial instructions and are the same model.
What made them different?
What made them different was the way they had been built, even if they had been constructed based on the same instructions, even in wildlife it does not matter if it is the same especie animals may be different and have multiple traits that can benefit or contribute to to their elimination.
Did all of the offspring survive the second trial enough to reproduce to the third trial?
Not all of the offspring survived the second trial, many of them didn't make it, in an ecosystem if a species population goes down the amount of offspring will also go down, and in this case if many didn't pass the second trial the population wouldn't increase as much as it would if all of them had passed.
What "variations" helped those survive and reproduce?
Some characteristics that helped to he survival of many of the planes that reached the third trial was the size of the wings, as well as the overall size of the whole plane, the grip and the way it was thrown, most of the time, what determined most of the time whether the plane passed the trial or not was luck, that is also a real life subject when talking about a ecosystem and all the species inside it. In nature many of these characteristics take a meaning when hunting is involved, when the predator is looking for the prey, the way it camouflages and it advantages when fighting for survival.
What outside factors affected the plane while testing them?
Some outside factors that affected the survival of the planes was the distance and the objects around it, in which many people will crash their planes against, this is similar to what really happens in a environment because many objects and disadvantage can cause the survival of a specie.