I also wonder how they handle merge requests in a monorepo, especially when it comes to the code review process.
In general it was pretty rare, in my experience. The code bases were pretty well modularized.
Are they split into git-sub modules/ frameworks? Curious to know the development code review flow.
In our PR tooling, there's nothing that enforces/encourages scoping changes to a specific subset of any given repo. We generally encourage smaller PRs as a best practice, but a huge chunk of the benefit of a moonorepo is folks can make cross-cutting changes if/when necessary.
We have some custom goo on top of Github that manages the review flow. Specifically, we try to make it easy for folks to farm out reviews to the teams that own the code you're editing, so they don't have to hop in Slack and track down reviewers.