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).
I used to work on Jira and it frustrates me seeing people say we don't dogfood our products when internally we do everything on a staging instance. jira.atlassian.com is not something developers use, and is a public facing instance.
I'm actually going to look into shorting Atlassian now.