根目录中的 snap 文件夹很重，而 home 中的 snap 文件夹很轻

Indeed, according to the answer of this post, all the configuration files of my snaps should be in the snap folder stored in home while there should only be mount points in the one stored in root. However, it seems to be the other way round [...].

This point is correct, but you're neglecting the fact that du doesn't care that something is mounted there. You asked it to tell you how big the files are in a specific location, and it told you. Check the output of the mount command and you'll notice that nearly everything in the /snap directory is just a mount point. Everything else in there is just a symlink. What you're seeing is correct, don't worry! You don't want to swap anything.

Q2: I also have a folder snapd in /var/lib which is 7gb. Is this normal?

是的，这是单个快照的存储位置（从它们安装到/snap/某处的位置）。这是正常的。

Q3：我注意到每个快照都存储了两个版本：我可以安全地删除旧版本以节省空间吗？

你可以; snapd 保留旧快照的原因是以防您需要恢复到旧快照。您可以使用snap list <snap name> --all. 如果您确信不需要旧版本，则可以使用sudo snap remove <snap name> --revision=<snap revision>. 请注意，您可以更改 snapd 配置以保留不同数量的快照：

$ snap set system refresh.retain=<number>