Apparently the repository is arranged so that e.g. the
6/ directory points into
6.<latest>, as long as that major release is supported. For example, when 6.10 was the latest 6.x version,
6/ would be the same as
6.10/. When 6.11 was released,
6/ changed to point into
6/ is probably actually just a symbolic link that points to whichever
6.x/ directory is the latest one, although the web listing won't tell you that.
When a particular major release has reached End-Of-Life (currently all 5.x and older), the content is replaced with a simple
readme file indicating that if you need packages for obsolete releases you should find them in
vault.centos.org system also does not allow downloading any
*.iso files to save bandwidth, although you can see they are there. If you need
*.iso files for old releases, you should use one of the CentOS vault mirror servers listed on the vault page, preferably one that's physically nearest to you.
The buildlogs.centos.org site, as the name implies, contains an archive of CentOS binary packages, the corresponding source code packages, and package build logs. These are not required for mirroring a basic RPM package repositories. The names of
c7-*/sub-directories reflect the various optional sub-repositories.
Unless you are troubleshooting a particular bug or working for CentOS Quality Assurance, they are not particularly interesting: if you don't know why you would need them, you probably should not download them. If you do need them, then you should put them where you need them.
If you are maintaining a CentOS repository mirror, you should read the CentOS mirroring guidelines to understand the best way to do it. It also mentions the address of a CentOS-mirror mailing list, and the Freenode IRC channel #centos-mirror: you could ask there to reach people that are likely to have the best knowledge about CentOS mirrors.