Regarding the external domain thing, I can say that dealing with domains in a big company gets about as bureaucratic and terrible as just about everything else; I experienced this myself - at a youngish company when I needed a new sub-domain off the big official domain, it was just talk to $dude on the DNS team and he’ll help you out. And he did. A few years later once things had “grown up” a bit, I needed to update a record and I asked the same guy. He told me I needed to fill out a 25 question form and they’d review it. I about half copy and pasted it from another team member’s project and they accepted it.

Obviously it doesn’t excuse the practice, but I can see why people use alternative domains to get things done. The above anecdote was also purely within the company; I’m sure that if you add in a partner/managed service, it only amplifies the complexity.