Setup /etc/yum.conf
“man yum.conf” is useful to learn all the options

Yum repos are located under
/etc/yum.repos.d

$releasever is decided by distroverpkg in /etc/yum.conf
distroverpkg=redhat-release is set as default
how to get value of distroverpkg
yum whatprovides redhat-release

how to change $arch
man setarch

Yum with multiple repos
http://blog.chrisramsay.co.uk/2009/08/14/yum-with-multiple-repos-yum-plugin-priorities-on-centos/