Looking at the Downgrading document linked above, it seems clear to me that where updates go is confusing, and I’ve seen no formal documentation of the method. I think the forum has some comments about searching until some place that can be written is found (good idea) but root and SYSTEM can write just about anywhere. Instead of looking for all the commentary on that, I’ll just point to one I found that’s actually a part of the larger question of updates. Duplicati is nice when it works. See Updating via web interface vs installing .deb package