To find out to which RPM package a file belongs:

saladin@adnlt714-vm2:~> rpm -qf /etc/hosts
setup-2.8.71-7.el7.noarch

Normally, all YUM repositories are stored in files /etc/yum.repos.d, but sometimes there are additional repos form somewhere (not really sure where these repos are really defined). So to get a complete overview about all the repos:

# To get a list of all repositories
  yum repolist

# To get more information
  yum repoinfo reponame

YUM repositories work a lot with caches, so when you search for a package and don't find it, make sure to delete the YUM cache:

yum clean all