By default, unless the /etc/cpupdate.conf file has been manually adjusted, cPanel does update the system packages on the server as well as part of the nightly update.

Does WHM update cPanel ? They are the same thing, so yes

Does it update the Host OS? Yes

Quoted from john on Tuesday, 10 June 2025

If that is true then all that needs to be done is a simple reboot every so often?

False, and this is the problem with today's "system admins". Relying on a control panel to do things like this is just amateur hour , and it's asking for trouble. There is way more to do on your server, from an administrative standpoint.