I don't understand why gitlab/github/bitbucket don't provide better tools for monorepo. This is a topic pretty trendy. But there is absolutely no tools helping with control access, good ci, ...
This becomes very critical for doing reviews, since it allows you to "trace" things without running them, apart from many other things. For example large scale refactorings looking for usages of functions, and other examples like it.
Why githab/gitlab/etc. can't do it? Well because hardly there could be one encompassing BUILD system to generate correctly this index.
I've been thinking about a tool like this for a long time. A way to attach to each commit not only the diff in the code, but also the list of places affected by the changes (usages of functions that are modified for example). Then during review we wouldn't have only a stupid diff. We would have a list of place to check to be sure that the changes make sense in the context of the project.
That by itself is another BIG PLUS for mono-repo (and "mono"-rules) - things are done one (opinionated) way, trunk based development - but thus giving you things that you won't be able to have normally.
Now indexing source file is not an easy and cheap task - it's basically a huge MapReduce done over several hours (just guessing), so there must be a reason for this to be done.