That's why I suggest asking in the forum if in doubt. I'm not sure if there is any wiki page explaining which archive is for what.
In short:
Root directory contains offline Qt installers "Single" directory contains whole Qt source code (all modules) combined. Zip is for Windows, tarball for Unix, Linux, Mac "Submodules" directory is for code of single Qt repositories. Unfortunately, they do not represent Qt modules directly. As said, that is due to historical reasonsLastly, Qt code can also be cloned from git, of course. Then checking out a given Qt version is just a matter of checking out a tag with version name.