I just beat the game. I'm gonna get no sleep but it was worth it. I'm pretty confused though.
What was the point of Project YoRHa? What was the point of the endless fighting?
Project YoRHa had basically two goals. The main goal was to establish and secure the lie that the Council of Humanity and the alleged human survivors on the moon were a thing, so that the androids would have a flag to rally around, a cause to fight and die for, as it was established that they were suffering from low morale due to the humans dying off thing. The secondary goal was R&D to create even more advanced androids for future war purposes. The purpose of the endless fighting was... meaningless, as Jackass puts it in her Machine Research Report summary. Both sides are essentially fighting just for the sake of fighting, as both sides have no other purpose, though it does clearly push both sides to evolve further, especially the Machine Network.
Why was an ark holding everyone's memories sent to space? Does this happen in every cycle too?
There is no real repetitive cycle here. The Androids and Machines have been fighting on and on for thousands of years, this being the 14th Machine War, but the events are unique. The Machines build the Ark because at least some part of the kind of fractious Machine Network has gotten over the need for fighting being the purpose of their existence and they want to fuck-off to a whole new digs so they don't need to fight anymore.
What did the original Devola and Popola do to get shitted on so badly? All I remember from Nier is that they were androids created by humans and were ultimately in the Replicants' side.
They were a big factor as to why Project Gestalt went tits up, with a little help from the implacable Papa/Brother Nier hisself. As in, a major factor in nailing the coffin shut on the subject of total human extinction.
What exactly happened in the ending? Weren't their memories being sent to space? So do they instead retain them and are allowed to live their lives? Is the war even over at the end? I know 2B and the others will know it's all bullshit but that's not gonna stop machines from attacking them.
From the Android side, only 9S had even the chance to basically have his consciousness join the Machine joyride to deep space. The Ark was a Machine project. And ending E? The Pods 042 and 153 decided that the planned for ending of all YoRHa, aka the total deletion of everything related to it, wasn't agreeable for them and they backed up and restored the data of the dynamic trio and rebuilt/repaired them. And no, going by the timeline given in the Emil Head weapons, the war did not end and the Machine Network will once again rear its head about 500 years from where the game ends, displaying new weapons tech. But there is some room for interpretation there.