What the guy above said is not true. Jira Cloud uses Jira Cloud to manage their projects.
a) the public facing jira.atlassian.com is Server/DC instance - but this instance is only used for customer/outside world facing tickets (I think)
b) internally for our own development we use a few (several?) cloud instances -> but I can only speak to those I interact with (Conf Cloud and Jira Cloud primarily).