Google doesn't use git, so a lot of the tools and features you might be used to don't work there. That's good, though. Developers and bots make changes to Google's code base at a very fast rate (many changes per second!), so there's no way that a central hook registry would be able to run everything necessary for every change, nor could a central pipeline take care of everything in the repository 😱.
Each directory has the equivalent of hooks (presubmit checks) that are checked using independent microservices (often maintained by independent teams) and must be run before the code is merged in head. There is also infrastructure for watching sub-trees, so tools and pipelines can be triggered when code is updated - often creating a cascade of even more changes!
You can compare such large-scale monorepos to the Internet. It gives us the illusion of having a single domain namespace, but DNS TLDs are responsible for sub-parts of it. Same for IP addresses. In Google's piper, directory trees have their own hooks, ownership and tools. It works beautifully, actually, but there's also a lot of complexity and technical debt to manage since Google isn't that new.
I worked at Google for a number of years and I'm happy to answer questions if you have any. Their source code management practices have been published publicly, so I wouldn't be disclosing anything new. Happy coding!
The point here is having good tooling for a use case. Git isn't the right tool for a monorepo if you want those other features (and most non-small development orgs will probably need). Sorry for all the shoring-up (most, probably). Every use case is different.
This also leads into writing good tools and defining good requirements/use cases but now I'm getting ahead of myself.
Git in this case acts just like a smart file storage. Microsoft and Facebook use git for very large repos and it works for them, for example.
Facebook uses Mercurial. Microsoft did a lot of work in the last year or two to make git work well with big repos.
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