Isn't it obvious that avatar is in the far distant future on another planet? This is likely because earth was deemed uninhabitable by mass-disaster and/or hyper-advanced war. The planet present in the Avatar series was likely inhabited because during the final days of earth, there were those who saw the impending doom, and as a result, launched FTL ships into space towards the planet in Avatar so that one day, humanity may live one. These ships would be full of human biological material and Adams/Eves so that humanity may restart. Bending and all the spiritual bullshit is made possible by long-forgotten technology present in all earth-based living organisms due to experimentation and augmentation before and during the war which destroyed earth. The "Spirit world" is just a VR(virtual reality) environment programmed before the mass-destruction of earth by those who saw what was coming so that future inhabitants of the new planet can have a collective domain of information accessible at all times. A domain that would give both spiritual and intellectual satisfaction to the future generations of earth. Its servers are located on the same ships that transported humanities seed to the new planet and were strategically programmed to enter deep orbit and cloak themselves so that even when we re-advance into space, we will not discover the truth. The "spirits" are artificial drones that are capable of changing their appearance through the use of nano-technology and hard-light. These drones programmed with highly advanced A.I. that are tasked with moderation humanity and preventing humanity from repeating itself and developing the same weapons that once destroyed earth. They also aid with the advancement of humanity as a whole by inspiring the creation of various technologies as humanity advances. The self-replicating nanotech present in all benders is linked to the VR domain and is accessible through it to most(but not all) benders. The story of Wan and the original benders is a lie programmed into the VR domain by those who saw the coming dark days of the first earth. A false origins story was created so that the truth behind our hideous past would not be discovered and so that our destructive weapons and world would be forgotten. They strategically designed the nano-tech that makes bending possible and divided it into elemental divisions. These divisions were created to prevent discrimination of race of race and religion so that nations may be formed based off of their elemental backgrounds and not their ethnicities and by doing so, minimalizing discrimination. The avatar line in order to promote unity among the nations and give the four divisions a symbol to rally behind towards progress throughout all their generations. The personalities and experiences of past avatars are recorded and after their deaths, duplicated as advanced A.I. so that they may serve as a connection to past wisdom for the current avatar so that they may lead the world towards progress. Of course, some things regarding the past avatars lives are rewritten by A.I. to make them seem more wise and less fallible. I believe I've covered most, if not all, plot devices in the series. Feel free to ask any questions regarding this post and I will try to answer.