I think the question is not very good.
For shared hosting and reseller they mostly use Cpanel/WHM but they also use the root account to secure, tweak the server. Or they hire a server management company to do that for them.
But it's not like you take your Cpanel server and you never worry about it.
It depends. Some use a control panel such as cPanel, Ensim, Plesk, HSphere, etc. Some manually add accounts from a root login,. Some have various levels of automation scripts that were built by their own staff. There is no one answer, because depending on the size of the company, the number of servers they manage, the number of accounts they host, and how fast the company grew, one solution may or not be better than another.