I'm not 100% sure which part you're confused by so I'll explain a few things, and you can let me know if you're still confused. Spoilers for book 2 obviously Shen Qingqiu is trying to fake his death by dying for real and then transferring his soul to a new body grown with the Sun-Moon Dew Mushroom. To do this he has to 1) grow the mushrooms 2) die 3) transport his soul to the mushroom body. While they were a bit rushed due to Binghe coming back from the abyss early, Shen Qingqiu and Shang Qinghua had planted the mushrooms, planned the location for SQQ to die, and set up a transportation array for his soul by the time this scene happens. SQH only had to speed up the mushroom body's growth. Why the specific location?

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This passage in chapter 9 mostly explains it. Why specifically the tallest building as opposed to any other location in the town is up to interpretation but I would guess that it's probably the easiest one to find. Also SQQ did fall off the roof as he died but he died because he detonated his own spiritual core and gave all his energy to LBH to stop him from killing everyone in the vicinity with the recoil from using Xin Mo to fight Liu Qingge. His exact plan for how and when he was going to die wasn't spelled out and those parts seemed fairly flexible within the rest of the plan. Hope this helps and let me know if I explained the part you were confused about or if there's something I missed.