I found the mention of most of Google’s code stored in a mono repo to be pretty crazy. https://dl.acm.org/doi/pdf/10.1145/2854146
It worked far better than you'd think. The ability to atomically change massive chunks of Google's code across projects was amazing. At some companies I worked at, if you wanted to make a breaking change in a common library, like say rename a method, it'd be a serious issue. You'd need to release a new version of your library, then you'd start migrating teams using that library one by one or cajoling them into it, an…
Total anecdote and not worth much, but I've seen it enough times to make note of it.