The majority of Google's code is in one gigantic repository. Sometimes there are some other repositories for very sensitive code, for example if you have a hardware vendor's closed-source driver code and they don't want many people to look at it. A few projects are in different repositories for their own reasons, like major open source projects. I don't think there's a single clear criteria you could come up with for what doesn't go in the main repository.
There are dependency management tools that help enforce public/private code on a wider scale and that help the build tools make sense of it all. There are also ownership tools that say what people and teams are qualified to review code in certain directories. The config files for all these tools are checked into the repository.
There's no versioning though. If you want to change an internal API you just update it and all the callers at once: patches in source control are already atomic. For truly massive changes (more code than most companies have) this gets too unwieldy and there are special tools and strategies people use.
This means you can't have a project depending on an out of date (internal) library. Without that requirement, you don't have the situation where different libraries need to be synced to different versions. And without needing to sync different things differently, you can get away with just one repository.
I worked at Amazon previously, which has world-class tools for dealing with versioned libraries in bulk, and Google's approach is vastly better. You spend less time worrying about breaking other people's dependencies, and you don't have someone spending a day fixing libraries every couple of weeks.
There are dependency management tools that help enforce public/private code on a wider scale and that help the build tools make sense of it all. There are also ownership tools that say what people and teams are qualified to review code in certain directories. The config files for all these tools are checked into the repository.
There's no versioning though. If you want to change an internal API you just update it and all the callers at once: patches in source control are already atomic. For truly massive changes (more code than most companies have) this gets too unwieldy and there are special tools and strategies people use.
This means you can't have a project depending on an out of date (internal) library. Without that requirement, you don't have the situation where different libraries need to be synced to different versions. And without needing to sync different things differently, you can get away with just one repository.
I worked at Amazon previously, which has world-class tools for dealing with versioned libraries in bulk, and Google's approach is vastly better. You spend less time worrying about breaking other people's dependencies, and you don't have someone spending a day fixing libraries every couple of weeks.