The tool that actually installs and removes RPM packages is /bin/rpm.

It is used by /bin/yum, which handles the network connections to repositories and the resolution of dependencies. Yum has a plug-in for subscription-manager (/usr/lib/yum-plugins/subscription-manager.py*) which contains the "secret sauce" needed to connect to RedHat repositories that require a particular subscription.

Subscription-manager, in turn, includes /usr/sbin/rhsmcertd, a daemon that maintains the entitlement certificates that allow access to subscription-only repositories, two D-Bus-based services /usr/libexec/rhsm-facts-service and /usr/libexec/rhsm-service (both started on-demand by systemd in RHEL7), and a daily cron job /etc/cron.daily/rhsmd that notifies you if your subscriptions are about to expire or have other errors.

Then there is /sbin/rhnsd, the "Red Hat Update Agent", essentially a Spacewalk query daemon, which periodically contacts Red Hat servers (or your Satellite if you have one), finds out if there are any updates, and optionally uses yum (and ultimately rpm) to actually install them for you.

If you're still using an older version of the Satellite, the rhn-client-tools RPM includes some tools for it; mainly /usr/sbin/rhn-profile-sync.

I guess rhnsd is the closest thing to "the actual service" you're looking for?