each_names = ([f'"{self.tree.item(iid)["text"]}",' for iid in self.tree.get_children(_id)] for _id in self.tree.selection())

読みづらいですか?
どう書いたらサイコー?

やっぱり丁寧な方が強いですか
each = []
for _id in self.tree.selection():
__names = []
__for iid in self.tree.get_children(_id):
____name = self.tree.item(iid)['text'] # string
____names.append(f'"{name}",') # to csv
__each.append(names)