Most big companies (think Google, Facebook scale) use a mix of Perforce [0] and Mercurial [1]. BitBucket supported Mercurial repositories up until recently, when they got rid of it, so now there's no super easy hosted Mercurial solution.
> Google doesn't use Perforce anymore. It's been replaced with Piper, you can read about it in articles from about 2015 or so. Perforce didn't scale enough.
Sorry, that's not true. I have a few friends who currently work at Google and I talked about remote work with them just recently. They all confirmed that Google uses Perforce.
[0]: https://www.perforce.com/
[1]: https://www.mercurial-scm.org/