PR's are not split into submodules/frameworks for distinct review purposes. Functionally though, right now we have three distinct monorepos (not very 'mono' but we're working on it!), that represent our three main development stacks.

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.