It's a monorepo, which is a bunch of libraries (in this case, the code for most Google products) in a single repository. Those libraries can have dependencies on each other.

One is a framework called Wiz, which renders the frontend for a bunch of Google web apps. You can imagine that the Wiz team might want to refactor an API, but not have to worry about different apps using different versions. In a monorepo, they can just find all the callsites and update them in the same commit that makes the API change. There's no package.json in google3 - everything builds from HEAD. Therefore, the commit that makes a breaking change is also the commit that fixes the would-be breakage.

This architecture evolved. Google used to use Perforce, which was a common commercial version control system before Git. Google had to figure out how to express the dependencies between packages in the monorepo (which can be in different languages with different build tools). They eventually created Bazel, which expresses those dependencies and orchestrates their build tools.

Build orchestration took a few attempts. Google3 is the third version of the monorepo, that is, the one that uses Bazel for dependency management.