Entire google ecosystem is over 80TB of code, 2 billions LoC. On one single repository. Same goes to facebook. Windows’ monorepo is 300GB of code.
And that’s just scratching the surface. They just keep reinventing every single tool or framework because appearently no one fucking knows how to git init.
The monorepo is more about dependency management. It’s a bigger hurdle at sufficient size to ensure everything is the right version then have a slightly unorthodox repo.