Not to diminish package managers, I don't think they are having to be quite as clever as Windows update, at least historically. I could be wrong, but I think they are more like dependency tree managers that also download and execute new installers.
No, they aren't clever. They're practical. There's no such thing as execution here. Packages are archives containing the entire files not differences. The manager downloads it, unpacks it, and adds/removes/replaces the package's files in the system.
They can also execute post-install or post-update scripts. And indeed, Linux package managers aren't as fancy as Windows Update is, but that fanciness isn't something I miss.
My distro's package manager probably breaks my system if it loses power during an update. Even so, it's been one of the more reliable and pleasant to use programs I've encountered.
