One thing to note is that Chromium and other Google projects are HUGE codebases; most codebases of people thinking about multi- or monorepos, microservices or monoliths, etc only WISH they had the scale of code and developers that companies like Google have.
All I'm saying is, beware of cargo cult thinking. Do something because you need it, it's practical, it's faster, not because someone else does it. I've had a few projects that were intentionally difficult because Someone decided to make it a microservices architecture, but in a monorepo to encourage the distributed monolith idea.
To be fair, Google, Facebook, et al chose to have monorepos when they were startups, so it's not necessarily cargo culting to look at what they did. I'd argue part of the success was cultural - with everything in one repo there isn't generally specific folders which are restricted in these large codebases, and that helps with a bunch of things.
All I'm saying is, beware of cargo cult thinking. Do something because you need it, it's practical, it's faster, not because someone else does it. I've had a few projects that were intentionally difficult because Someone decided to make it a microservices architecture, but in a monorepo to encourage the distributed monolith idea.