Facebook uses mercurial across the board, at least from what I saw a couple years back. A good chunk of the large file / large repo has been open sourced as EdenFS [1] which uses file notifications to update the status as you make changes to amortize the cost so that it’s already computed by the time you query (watchman is integrated). That being said, very few code bases grow to this size unless you are a major tech company and have a single mono repo (with a few major OSS projects as notable counter examples).

[1] https://github.com/facebookexperimental/eden