I think you were misunderstanding something. Why would every code change cause a compile and test across the entire company? That is to say; not only does that not scale, it’s totally unnecessary*. Only the downstream consumers of a change are rebuilt and tested, like you’d expect (see: bazel and the monstrous makefile before that). In this sense, the fact that Google uses a monorepo is mostly an implementation detail. It has some impact on the company’s workflows and tooling, but not its software architecture.

* unless you’re changing a very common dependency, of course, and Google has tooling for this.