Finished doing the second half of the game for the second ending. I really loved exploring that area and it's wild how all that stuff is just missable. Does make it more special when you do discover it. There were some nice lore tidbits but i wasn't really paying attention much to the diaries scattered around. The final boss of the second ending was much easier though, since she stays on the ground most of the time you can easily abuse patchouli's side magic on her. Though it is still easy to die if you're not careful. The extra dungeon giving me 2 more hearts definitely helped.
SPOILERS AHEAD
From what i can remember of the journals, the story is that yukari yakumo made the hakurei barrier by sealing Sakura, the "first" daughter of hakurei, but then for some reason ww2 made sakura feel a lot of pain so yukari tried some experiments to ease it but it failed horribly and her comrades died on the way. And i guess.. she just left her there? And in the ending cutscene, did we just destroy the hakurei barrier? Pretty wild if that's the ending. It will be a hell of an incident for Reimu to solve if so. Though i don't know how the mask of emotions connects to all of this.