A bunch of larger projects have a mirror on GitHub for easier access.
BTW there's launchpad.net which is often overlooked, bit it's vital for Ubuntu-specific projects.
At paid day jobs, I had to use BitBucket at least twice, and I miss better code review tools, like Phabricator.
GitHub definitely dominates the market, partly due to the network effects, but I don't think they have a lot of moat. If something goes badly enough wrong there, there will be plenty of viable alternatives with an easy to trivial migration path.
Their moat is a billion development tool vendors that have "integrate with Github" as a must-have and expected functionality.
It's overlooked because in true Canonical fashion they went hard in on their not-invented-here-syndrome VCS that nobody asked for or wanted. That and also the integration with Ubuntu and nothing else.
There is also a lot of stuff on Gitee (China), but due to langauge barrier, it's hard to judge.
That doesn't sound true to me at all, except maybe in some very small niches. I've used Bitbucket at exactly one job; I've found Codeberg, but no project I've used was actually hosted there; and literally everything else I see or use is on Github.