Google has a CI infrastructure which runs most of the affected tests for each commit (which they call "CL") on thousand of machines in parallel. Though even for Google, running the entire test suite every time is prohibitively expensive so they have a way to merge and run multiple CLs in a single batch run every 3 hours, which is useful for testing a CL that may affect hundreds of thousands of build/test targets. If you're interested, this paper may give you an idea how Google is doing test.

https://static.googleusercontent.com/media/research.google.c...

> Given the size of a monrepo, is it possible to run the entire test suite in one's development environment, or do they have another endpoint to push to to run tests on a dedicated server?

Eventually you hit a point where you need systems to run the tests for you. Making this work is part of the investment in infrastructure and tooling you need to do as a big serious company.